How It Works Book of the Human Body - Digestion and Nutrition

THE HUMAN BODY How It Works

Digestion and Nutrition

THE HUMAN BODY How It Works

Cells, Tissues, and Skin The Circulatory System Digestion and Nutrition The Endocrine System Human Development The Immune System The Nervous System The Reproductive System The Respiratory System The Senses The Skeletal and Muscular Systems

THE HUMAN BODY How It Works

Digestion and Nutrition

Robert Sullivan

DIGESTION AND NUTRITION Copyright © 2009 by Infobase Publishing All rights reserved. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For information, contact: Chelsea House An imprint of Infobase Publishing 132 West 31st Street New York NY 10001 Library of Congress Cataloging-in-Publication Data Sullivan, Robert J. (Robert James) Digestion and nutrition / Robert Sullivan. p. cm. — (The human body: how it works) Includes bibliographical references and index. ISBN 978-1-60413-367-7 (hardcover) 1. Digestion. 2. Nutrition. 3. Digestive organs. I. Title. QP145.S86 2008 612.3—dc22 2008030741 Chelsea House books are available at special discounts when purchased in bulk quantities for businesses, associations, institutions, or sales promotions. Please call our Special Sales Department in New York at (212) 967-8800 or (800) 322-8755. You can find Chelsea House on the World Wide Web at http://www.chelseahouse.com Text design by Erika Arroyo, Erik Lindstrom Cover design by Ben Peterson Printed in the United States of America Bang FOF 10 9 8 7 6 5 4 3 2 1 This book is printed on acid-free paper. All links and Web addresses were checked and verified to be correct at the time of publication. Because of the dynamic nature of the Web, some addresses and links may have changed since publication and may no longer be valid.

Contents

Introduction Denton A. Cooley, M.D. President and Surgeon-in-Chief of the Texas Heart Institute Clinical Professor of Surgery at the University of Texas Medical School, Houston, Texas

1 2 3 4 5 6 7 8 9

6

Introduction to Digestion and Nutrition

10

Nutrition and Major Nutrients

12

Minor Nutrients and Metabolism

23

Digestion, Absorption, and Elimination

32

Chewing and Swallowing

41

The Stomach and Small Intestine

52

The Large Intestine and Elimination

67

Common Health Problems

76

Guides to Healthy Eating

80



Appendix: Conversion Chart

98



Glossary

99



Bibliography

110



Further Resources

111



Picture Credits

114



Index

115



About the Author

120

Introduction The human body is an incredibly complex and amazing structure. At best, it is a source of strength, beauty, and wonder. We can compare the healthy body to a welldesigned machine whose parts work smoothly together. We can also compare it to a symphony orchestra in which each instrument has a different part to play. When all of the musicians play together, they produce beautiful music. From a purely physical standpoint, our bodies are made mainly of water. We are also made of many minerals, including calcium, phosphorous, potassium, sulfur, sodium, chlorine, magnesium, and iron. In order of size, the elements of the body are organized into cells, tissues, and organs. Related organs are combined into systems, including the musculoskeletal, cardiovascular, nervous, respiratory, gastrointestinal, endocrine, and reproductive systems. Our cells and tissues are constantly wearing out and being replaced without our even knowing it. In fact, much of the time, we take the body for granted. When it is working properly, we tend to ignore it. Although the heart beats about 100,000 times per day and we breathe more than 10 million times per year, we do not normally think about these things. When something goes wrong, however, our bodies tell us through pain and other symptoms. In fact, pain is a very effective alarm system that lets us know the body needs attention. If the pain does not go away, we may need to see a doctor. Even without medical help, the body has an amazing ability to heal itself. If we cut ourselves, the blood-clotting system works to seal the cut right away, and the immune



Introduction

defense system sends out special blood cells that are programmed to heal the area. During the past 50 years, doctors have gained the ability to repair or replace almost every part of the body. In my own field of cardiovascular surgery, we are able to open the heart and repair its valves, arteries, chambers, and connections. In many cases, these repairs can be done through a tiny “keyhole” incision that speeds up patient recovery and leaves hardly any scar. If the entire heart is diseased, we can replace it altogether, either with a donor heart or with a mechanical device. In the future, the use of mechanical hearts will probably be common in patients who would otherwise die of heart disease. Until the mid-twentieth century, infections and contagious diseases related to viruses and bacteria were the most common causes of death. Even a simple scratch could become infected and lead to death from “blood poisoning.” After penicillin and other antibiotics became available in the 1930s and 1940s, doctors were able to treat blood poisoning, tuberculosis, pneumonia, and many other bacterial diseases. Also, the introduction of modern vaccines allowed us to prevent childhood illnesses, smallpox, polio, flu, and other contagions that used to kill or cripple thousands. Today, plagues such as the “Spanish flu” epidemic of 1918–1919, which killed 20 to 40 million people worldwide, are unknown except in history books. Now that these diseases can be avoided, people are living long enough to have long-term (chronic) conditions such as cancer, heart failure, diabetes, and arthritis. Because chronic diseases tend to involve many organ systems or even the whole body, they cannot always be cured with surgery. These days, researchers are doing a lot of work at the cellular level, trying to find the underlying causes of chronic illnesses. Scientists recently finished mapping the human genome, which is a set of coded





digestion and nutrition

“instructions” programmed into our cells. Each cell contains 3 billion “letters” of this code. By showing how the body is made, the human genome will help researchers prevent and treat disease at its source, within the cells themselves. The body’s long-term health depends on many factors, called risk factors. Some risk factors, including our age, sex, and family history of certain diseases, are beyond our control. Other important risk factors include our lifestyle, behavior, and environment. Our modern lifestyle offers many advantages, but is not always good for our bodies. In western Europe and the United States, we tend to be stressed, overweight, and out of shape. Many of us have unhealthy habits such as smoking cigarettes, abusing alcohol, or using drugs. Our air, water, and food often contain hazardous chemicals and industrial waste products. Fortunately, we can do something about most of these risk factors. At any age, the most important things we can do for our bodies are to eat right, exercise regularly, get enough sleep, and refuse to smoke, overuse alcohol, or use addictive drugs. We can also help clean up our environment. These simple steps will lower our chances of getting cancer, heart disease, or other serious disorders. These days, thanks to the Internet and other forms of media coverage, people are more aware of health-related matters. The average person knows more about the human body than ever before. Patients want to understand their medical conditions and treatment options. They want to play a more active role, along with their doctors, in making medical decisions and in taking care of their own health. I encourage you to learn as much as you can about your body and to treat your body well. These things may not seem too important to you now, while you are young, but the habits and behaviors that you practice today will affect your physical well-being for the rest of your life. This book series, The Human Body: How It Works, is an excellent introduction

Introduction

to human biology and anatomy. I hope that it will awaken within you a lifelong interest in these subjects. Denton A. Cooley, M.D. President and Surgeon-in-Chief of the Texas Heart Institute Clinical Professor of Surgery at the University of Texas Medical School, Houston, Texas



1 Introduction to Digestion and Nutrition On the way home from her morning classes, Amy stops for lunch at a fast-food restaurant. She is in a hurry, and she knows that the meal will be served fast and that the food will be safe. The food may not be the tastiest in the world or very good for her, but it will get her through until dinner. Amy has eaten in this kind of place hundreds of times before. She orders a burger, fries, and a chocolate shake. She knows the burger and fries have lots of fat and salt that she does not need. She also knows the shake is risky for her. She has a form of lactose intolerance that sometimes results in abdominal cramping and diarrhea after ingesting milk products. But she is in a hurry, and at least she knows what she gets here; besides, she has been thinking about the chocolate shake all morning. After Amy eats her lunch, her digestive system processes the hamburger, fries, and chocolate milkshake into nutrients her body can use. This process takes the food we can see, smell, and taste and breaks it down, first physically into smaller and smaller pieces, and then chemically into nutrients that can be used by the cells. These nutrients are relatively simple chemicals that can pass through the cells in the walls of the digestive tract and into the blood, which transports them to all the cells of the body. 10

Introduction to Digestion and Nutrition Digestion is the physical and chemical breakdown of food into forms that can be used by the body’s cells. The process begins in the mouth when a bite of food is chewed, thereby breaking it down into smaller pieces and mixing it with saliva. The food mass is then swallowed. The food has been reduced to a smaller size, but it is still not small enough. Digestion continues in the stomach and intestines until the food particles are an appropriate size and the nutrients can travel to the body’s cells. The process by which nutrients pass through the walls of the digestive tract and into the blood is called absorption. The blood transports the nutrients to the cells of the body, where they are absorbed to provide energy and needed structural materials. As you read through the chapters, you will follow Amy’s lunch on its journey through her body. You will read about what is really in her food, how it is digested, or broken down, and how it is absorbed into the cells. The hamburger and fries she eats contain a lot of fat and salt, and the milkshake will most likely make her feel sick. You will also learn how this is related to her lactose intolerance. You will read about why we need nutrients. Why do we need a variety of carbohydrates, proteins, lipids, vitamins, and minerals? If we cannot absorb food until it is broken down into much smaller pieces, how does it get into the body? There is also a discussion of accessory organs that contribute to digestion, such as the liver and pancreas. Finally, this book will discuss some nutritional controversies and health problems related to the digestive tract and nutrition. Digestive anatomy and physiology are integrated as much as possible throughout the chapters. As you read about the anatomy of a specific portion of the digestive tract, the physiology, or the way this portion works, is discussed.

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2 Nutrition and Major Nutrients Why Do HUMANS Have To Eat? Humans and other animals need to eat because they need resources that their bodies cannot provide. Food provides the material for the structural needs of the body, such as replacing worn-out cells and tissues, building more muscle tissue, and growth. In order to fulfill these needs, the body also needs the energy that food provides. This energy is used to make and break chemical bonds in complex biochemical compounds, to hold these compounds together, and to change them. The digestive system and its accessory organs have evolved to supply individuals with the energy they need to work with these chemical bonds. There are three types of chemical bonds. An ionic bond forms between charged atoms where positive and negative charges attract each other. These bonds are fairly strong, but not so strong that energy is needed to alter them. A hydrogen bond is a type of weak chemical bond. It bonds substances together during chemical reactions or fine-tunes the structure of complex compounds so that they can function properly. Hydrogen bonds also exist between water molecules and anything mixed 12

Nutrition and Major Nutrients

in water. Hydrogen bonds allow the water molecules to support the compounds that are dissolved in the solution, but are weak enough to allow the compounds to diffuse through the water. Hydrogen bonds are so weak that the chemicals held with them can separate just by drifting off into the surrounding water. The third type of chemical bond, a covalent bond, requires energy to make or break it. Covalent bonds are formed when atoms share electrons. The electrons from two or more atoms are found in the space around the nuclei of the atoms, forming a tight bond, almost like a wall. Energy released by the breaking of chemical bonds must be put into a form that cells can use. The chemical energy used by cells is stored in the bonds of adenosine triphosphate (ATP), an RNA nucleotide. The three phosphates in ATP are attached to the adenosine in series so that the molecule looks like this: A-P-P-P (Figure 2.1). The phosphates are negatively charged and repel each other. Attaching the second and third phosphate requires energy to force the phosphates onto the molecule. The energy stored in ATP is the energy in the bonds that hold the repelling phosphates together. When the energy is needed, the third phosphate is removed, and the energy that was holding the phosphate onto the ATP molecule is used to make or break a covalent bond. The loss of the phosphate from the ATP produces adenosine diphosphate (ADP). ADP can become ATP by extracting energy from a nutrient and using it to attach another phosphate. These energy transport molecules function like rechargeable batteries, with the difference being that the energy is mostly discharged each time the ATP is used.

Types of Nutrients There are major and minor nutrients. Major nutrients—carbohydrates , proteins , and lipids —serve as energy sources or as building blocks for larger biochemical compounds. Minor nutrients, which include all vitamins and minerals , assist the chemical reactions that occur with major nutrients.

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DIGESTION AND NUTRITION

Figure 2.1 ATP is an energy–storage compound that contains three phosphate groups. Cells obtain the energy they need to carry on their life functions from the breakdown of ATP. When the bond between the last two phosphate groups is broken, the energy released can be used by the cell for anything from replication and division to making proteins and extracting nutrients from food.

A balanced diet includes all of the necessary major and minor nutrients. If the diet is not balanced, some energy sources or building blocks will be missing, and the body will not function properly.

Nutrition and Major Nutrients

Carbohydrates Carbohydrates, a group of molecules that include sugars and starches , provide energy for the body when the molecules are broken down. All carbohydrates contain carbon, hydrogen, and oxygen. They are categorized by size: monosaccharides ; disaccharides ; and polysaccharides , or complex sugars.

Monosaccharides Monosaccharides, such as glucose, fructose, and galactose, are simple sugars. Usually, the ratio of carbon to hydrogen to oxygen is 1:2:1 such that there is one carbon to two hydrogens to one oxygen. Most of the sugars used in the body are six-carbon sugars, so their formula is written as C6H12O6. The body’s sugar biochemistry is based on the breakdown of glucose. Fructose and galactose feed into the pathway of these chemical reactions. Disaccharides Two monosaccharides bonded together form a disaccharide. There are three types of disaccharides: sucrose, lactose, and maltose. Each one has glucose as at least one of its sugar units. Sucrose, which is made of glucose and fructose, is common table sugar. Lactose, made of glucose and galactose, is the sugar found in dairy products. Maltose, made of two glucose molecules, is found in anything “malted” and is also the sugar primarily used to make beer. Because disaccharides are too large to pass through cell membranes, they must be broken down into mono­saccharides before they can be used by the body. Polysaccharides Polysaccharides, or complex carbohydrates, are composed of many monosaccharides linked in a chain. There are two

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DIGESTION AND NUTRITION

types of polysaccharides of importance to the body: starches and glycogen. These are made up of glucose subunits and have slightly different forms, depending on their source and the types of chemical bonds holding them together. Both plants and animals use polysaccharides as a form of short-term energy storage. Starches are storage carbohydrates found in plants. There are two types of starch, depending on the complexity of the structure: amylose and amylopectin. Amylose is easily digestible and has a simple structure resembling a bunch of strings made up of glucose molecules linked together in a straight line. Amylopectin has a more complex structure, including a large number of crosslinkages between the strings, and is more difficult for the body to digest. Glycogen is the storage carbohydrate form found in animals. Glycogen is similar to amylopectin, but less complex. Polysaccharides must be digested to their individual glucose units for the body to be able to use the energy. Mono- and disaccharides are found in fruits, sugarcane, sugar beets, honey, molasses, and milk. Starches are found in grains, legumes, and root vegetables. Glycogen is present in all animals, although the primary source is beef. Polysaccarides in food take longer to digest and absorb than simple sugars or dissacharides. This longer time provides a steady supply of monosaccharides for absorption and metabolism. Simpler sugars provide a spike in absorbed sugars, which may lead to increased fat production. As mentioned earlier, carbohydrates are used for energy. When glucose is broken down, some of the energy released from the chemical bonds goes into the synthesis of ATP from ADP molecules. If carbohydrates are not immediately needed, they are converted to glycogen or fat and stored. If not enough glucose is available, the liver breaks down glycogen to release glucose. The liver can convert amino acids into glucose, a process called gluconeogenesis. If adequate sugar is not available in the diet, amino acids are converted to glucose instead of being used in protein synthesis.

nutrition and Major nutrients

Cellulose is a type of polysaccharide found in plants. It is

a major component of wood. It cannot be broken down into smaller units by the human digestive system, so it is not digestible. When we ingest cellulose, it is considered roughage or fiber. Although we get no nutritional value from cellulose, it binds cholesterol in the intestine and helps us eliminate this chemical. Fiber also helps to regulate the digestive tract and keep people “regular.”

proTeiNS Proteins have many functions in the body. They can be used for energy; they form many different structural parts, including muscle; and they are basic components of hormones and enzymes. Proteins are made of long chains of 20 different kinds of amino acids. The structure of proteins ranges from small and simple to large and very complex. The function of a protein depends on its structure. The chains of amino acids bend and twist to a three-dimensional form, depending on the sequence of the amino acids. In general, the structure and appearance of proteins can be classified as fibrous or globular. Fibrous proteins, which are strandlike in appearance, are called structural proteins. They include collagen, keratin, and the contractile proteins of muscles. Collagen provides strength to the tendons and ligaments that hold bones and muscle together. Keratin is found in the outermost layer of skin, where it “seals”

YOUR HEALTH: EMPTY CALORIES sometimes foods are described as having empty calories. this means that the item is made mostly of sugar, probably sucrose, and not much of anything else. When carbohydrates are ingested along with proteins, lipids, vitamins, and minerals, they form part of a balanced diet that fills our nutritional needs.

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DIGESTION AND NUTRITION

the skin surface, preventing evaporation of water from underlying tissues and keeping invading microorganisms out. It is also a major component of hair and nails. Contractile proteins of muscles allow muscle tissue to contract, or shorten. Globular proteins, which are compact, spherical proteins, have a wide variety of functions. Some are found in hormones, such as human growth hormone, which helps regulate growth in the body. Enzymes are globular proteins that increase the rate of chemical reactions in the body.

Proteins and Diet Of the twenty different amino acids, there are eight, called essential amino acids, that the human body cannot synthesize. These eight amino acids are tryptophan, methionine, valine, threonine, lysine, leucine, histadine, and isoleucine. Because humans cannot make them, they must be supplied in the diet. If they are not supplied, proteins cannot be made, which results in a protein deficiency. Protein deficiency during childhood can result in developmental problems that restrict both mental and physical development. Protein deficiencies in adults cause a number of problems, such as premature aging, problems in fighting infections, and bleeding in joints and the digestive tract. The most complete sources of dietary proteins are found in animal tissues because they contain all the essential amino acids. Plants can also provide amino acids, but you may have to eat more than one kind of plant food at one meal to supply all the essential amino acids at the same time. Nitrogen balance is an evaluation of the amount of protein in the body, and it is used to determine an individual’s nutritional status. In a healthy person, the production of proteins is equal to the breakdown of proteins, and the nitrogen balance is said to be neutral. In a person who is growing or repairing tissue damage and has adequate amino acid resources for protein production, the production of protein exceeds protein breakdown, and the

Nutrition and Major Nutrients

person is said to be in positive nitrogen balance. If proteins are being broken down faster than the body can replace them, the person is said to be in negative nitrogen balance, which is not good. Negative nitrogen balance means that the person needs supplemental proteins and amino acids to achieve a neutral or positive nitrogen balance.

Fats and Lipids Lipids are insoluble in water, and thus they are difficult to carry in the blood. The terms lipids and fats can usually be used interchangably. Lipids are frequently liquid, while fats are more solid at room temperatures. Lipids are categorized as triglycerides, phospholipids, and steroids. The principal dietary lipids in the body are cholesterol (a steroid) and triglycerides. Phospholipids are mostly tied up in cell membranes and do not play a significant role in energy metabolism. Triglycerides, which are made in the liver to store excess energy from carbohydrates, make up a major portion of adipose tissue. This tissue provides the body with insulation to keep warm and cushions joints and organs for protection. Triglycerides are composed of three-carbon glycerol molecules with a fatty acid molecule attached to each of the three carbons. The fatty acid portion of the triglyceride is the “fat” in the molecule and gives it the lipid characteristics. Fatty acids are chains of 12 to 30 carbon atoms. Attached to the carbon atoms are hydrogen atoms. If all the possible hydrogen atoms are attached to the chain, the fatty acid, or the triglyceride, is said to be a saturated fat. If any of the hydrogen atoms are missing, the fatty acid is unsaturated. These forms of triglycerides behave slightly differently in the body. Saturated triglycerides contribute more to the buildup of plaque in arteries and are considered less healthy than unsaturated fats. Saturated triglycerides are found in all animal tissues, and unsaturated triglycerides are found in nearly all plants, except those derived from coconuts or the palm tree. Just as there are

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essential amino acids that must be supplied by the diet, there are two types of essential fatty acids that must be supplied by the diet: linoleic acid and α-linolenic acid (α is the Greek letter alpha). Linoleic acid is also called omega-6 fatty acid and α-linolenic acid is called omega-3 fatty acid. About 90% of the body’s dietary fat intake consists of the fatty acids palmitic acid, stearic acid, oleic acid, and linoleic acid. Linoleic acid is found in vegetable oils, especially corn and safflower oils, and α-linolenic oil is found in rapeseed oil. Essential fatty acid deficiencies contribute to dermatitis, a depressed immune system, anemia, growth retardation, infertility, and cardiac, liver, and respiratory problems.

DID YOU KNOW? Fats are not soluble in water. thus, for the body to carry lipids, such as cholesterol and triglycerides, in the blood, which is water-based, the lipids are mixed with proteins that can dissolve in water. the proteins act as carriers for the fats. different proteins give different characteristics to these lipid-protein mixtures, which are called hDL (high-density lipoprotein) and LDL (low-density lipoprotein). neither one of them is good or bad. all dietary fats are needed by the body, just not in excess. If the fats separate from their protein carriers, they can no longer travel in the blood or mix well in cells. (this is analogous to the separation of water and oil in salad dressing.) In the blood, these floating lipids attach to fatty deposits called plaque on the walls of blood vessels (Figure 2.2). If the plaque becomes large enough, it can close off part of the blood vessel. If part of the plaque breaks off from the vessel wall, it can travel to capillaries, where it may get stuck and completely block the smaller vessel. When this blockage occurs in the blood vessels of the heart, a heart attack results. If this blockage occurs in the brain, a stroke results. LdL is assembled in the liver from proteins, cholesterol, and triglycerides and sent into the blood to deliver these fats to the body’s tissues. the lipids and proteins tend to separate, especially if there is an increase

nutrition and Major nutrients

TrANS fATS Some triglycerides contain what are called trans fats. These are unsaturated fatty acids that have some of the harmful properties of saturated fats. Although trans fats contain unsaturated fatty acids, the arrangement of the hydrogens in these molecules produces damage to the heart and blood vessels. The advantage of using trans fats in food products for commercial use is that they have a longer shelf life and allow prepared foods to stay fresher longer. It is estimated that Americans consume about a bit less than 6% of their fats as trans fats; this is slightly less than 3% of our caloric intake. The

figure 2.2 Plaque (yellow) blocking the aorta. extra fat in the body can accumulate in blood vessels and form plaque. this plaque can grow large enough to block the flow of blood through the vessel. Plaque that occurs in the major vessels of the heart can cause a heart attack.

in blood pressure, as in hypertension. thus, LdL has earned the name “bad cholesterol.” HdL protein is made in the liver and released into the bloodstream without any lipids. Its job is to scavenge cholesterol from the body’s tissues and blood vessels. When the HdL proteins are full of cholesterol, they are removed from the blood by the liver and the cholesterol is made into bile, a digestive fluid. Because HdL removes cholesterol from tissues and does not significantly contribute to the buildup of plaque, it has earned the name “good cholesterol.”

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hazard to consumers is that they think they are substituting unsaturated fats for saturated ones in their diets, but they are not. These fatty acids contribute as much to the clogging of arteries as saturated fats. Trans fats are most frequently found in prepared foods, such as cakes, cookies, pies, and breads. They also found in prepared meats and hard margarine. Steroids are another type of lipid that have hydrocarbon rings. Cholesterol, one of the most important steroids, is both made in the liver and ingested in food from animal sources. Plants have a counterpart to cholesterol called phytosterol, but this compound cannot be absorbed by humans and does not contribute to dietary fats. Cholesterol forms the chemical framework of steroid hormones. Testosterone and estrogen, which are reproductive hormones, are both steroids. Aldosterone, a steroid hormone from the adrenal cortex, assists in the renal conservation of sodium. Cholesterol is also found in cell membranes, where it makes them pliable. Its presence in the membranes of red blood cells allows the cells to squeeze through small capillaries.

Connections The body takes food and breaks it down into the nutrients it can use, both major and minor. The major nutrients include carbohydrates, proteins, and lipids. Vitamins and minerals are types of minor nutrients and will be discussed in Chapter 3. Nutrients serve as building blocks for larger chemicals and the energy that fuels all of the body’s processes, from cell repair to muscle contraction.

3 Minor Nutrients and Metabolism Although sugars, proteins, and fats receive a lot of attention in discussions of nutrition, vitamins and minerals also play a vital role in our diet. This chapter examines these nutrients and also includes a brief discussion of the way we actually extract energy from nutrients through biochemical pathways.

Vitamins Vitamins and minerals are classified as minor nutrients. These compounds are vital to the body, but are needed in much smaller amounts than carbohydrates, proteins, and lipids. Vitamins do not supply energy or serve as building blocks for other compounds. Instead, they work with the chemicals that make, modify, and metabolize the major nutrients. Vitamins are classified as either fat or water soluble. Fatsoluble vitamins are stored in the body and may reach toxic levels if a person ingests too much of them. These vitamins are absorbed the same way as other fats (see Chapter 6). The four fat-soluble vitamins are A, D, E, and K. (See Table 3.1 for details on these vitamins.) Vitamin D is made in the skin when it is exposed to ultraviolet light from the sun. Nutri23

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tional supplementation of vitamin D is usually necessary during childhood to ensure proper bone growth. Vitamin K is involved in the process of blood clotting. A common blood thinner taken after a heart attack or stroke inactivates vitamin K and decreases the blood-clotting factors from the liver. The decrease in clotting factors results in a lower tendency to clot and helps prevent a second heart attack or stroke. Vitamin K is found in many leafy vegetables, and it is also produced by the bacteria that inhabit the intestines (see Chapter 7). There are many water-soluble vitamins, including vitamin C and several B vitamins. Except for storage of vitamin B12 in the liver, none of the water-soluble vitamins are stored

Table 3.1 Important Vitamins vitamin

importance

problem excess

problem Deficit

A (Retinol)

Used for production of chemicals in vision

Neurological problems

Night blindness

D

Calcium absorption

Neurological problems

“Soft” bones

E

Antioxidant

Neurological problems

Damage from chemical free radicals

K

Production of blood clotting factors

Neurological problems

Bleeding, inability of blood to clot

C (Ascorbic acid)

Antioxidant

None—excess secreted in urine

Scurvy

B Complex

Energy carriers in metabolism

None—excess secreted in urine

Metabolism problems

B12

Participates in DNA synthesis

None

Anemia

Folic acid

Participates in DNA synthesis

None

Anemia

Minor Nutrients and Metabolism

in the body. Excess amounts of these vitamins are excreted in the urine. Vitamin C, also called ascorbic acid, is found in citrus fruits. Vitamin B12 is only found in meat, while folic acid is present in leafy vegetables. Other vitamins can be found in a variety of fruits and vegetables. Humans usually have about one year’s supply of vitamin B12 stored in the liver, but no extra folic acid. During pregnancy, women are especially prone to folic acid deficiency and need to take supplemental vitamins to help maintain the development of the fetus.

Minerals The body needs several minerals, including calcium, phosphate, magnesium, sodium, potassium, chloride, sulfur, and iron. The body also needs trace metals , including zinc, iodine, copper, manganese, fluorine, selenium, and molybdenum, in very low concentrations. Care should be taken if supplements are used, as metals such as selenium and chromium are toxic in excess. Calcium, magnesium, and phosphate provide strength to bones and teeth. Iron is important in hemoglobin and other oxygen-carrying compounds. Iodine is a vital part of the hormone made by the thyroid gland. Iodine deficiencies result in marked swelling of the thyroid gland and neck, producing a goiter. Individuals in the United States usually receive adequate iodine from iodized salt. Countries that do not add iodine to their salt, such as China, have a high incidence of goiter. Thyroid hormone controls the body’s metabolic rate. People with a deficiency of this hormone have a lower-thannormal metabolic rate, affecting growth and development in childhood and overall body metabolism in adults.

Metabolism Once nutrients have entered the body cells, they take part in a wide range of biochemical reactions. Metabolism is the

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Figure 3.1 Anabolism and catabolism are both metabolic processes. Anabolism is the synthesis of larger molecules from smaller ones, while catabolism is the breakdown of larger molecules into smaller ones.

sum of the chemical reactions that occur in cells. Metabolic reactions either make molecules or structures or break them down. Anabolism refers to reactions in which larger molecules are made from smaller ones; for example, the bonding of amino acids to make proteins. Catabolism refers to reactions in which large or complex molecules are broken down into smaller ones (Figure 3.1). Anabolic reactions usually require the addition of energy to proceed. Catabolic reactions tend to release energy from the compounds. The energy released from one reaction can run another reaction. Energy is extracted from compounds in two ways. When some chemical reactions occur, there is energy left over. This energy can be used to add a third phosphate onto ADP to form ATP, a process called substrate phosphorylation. A substrate is a compound being acted upon by an enzyme. Phosphorylation is the process of adding a phosphate group (PO4-) to an atom or group of atoms. This process accounts for relatively little of the ATP produced. The rest of the ATP is made by harnessing the energy of the electrons of hydrogen atoms.

Minor nutrients and Metabolism

These atoms are split, and the electrons are passed through a series of reactions resulting in a large amount of ATP. Oxygen is used in this process, but only at the end, when it receives an electron. The addition of two electrons to oxygen attracts two hydrogen ions (protons) from the surrounding medium, and the result is water (H2O). This second method of producing ATP is called oxidative phosphorylation. Because triglycerides hold a large number of hydrogen atoms, storing twice the energy of carbohydrates, fatty acids are much more efficient as energy storage molecules. The overall process by which energy for metabolism is extracted from nutrients is called cellular respiration. The reactions of aerobic cellular respiration require oxygen, while those of anaerobic cellular respiration do not require oxygen.

YOUR HEALTH: CALCULATING BMR to estimate the number of calories the body needs each day, the basic metabolic rate (BMr) must be calculated. 1.

Calculate body weight in kilograms (pounds divided by 2.2). Males should then proceed to the next step. Females should first multiply the kilogram figure by 0., then proceed to step 2.

2.

Multiply this number by 24. the result is the number of calories a person should burn in a day to maintain body weight. If the person eats less than this, the person will lose weight. If the person eats more calories than this, the person will gain weight. If a person changes their level of activity, is actively growing, or is recovering from an illness, the number of calories should be adjusted appropriately. table 3.2 on page 2 shows the number of calories burned through different activities.

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Figure 3.2 Glucose is broken down to carbon dioxide and water by some reactions of glycolysis, the Krebs cycle, and the electron transport chain. First, glucose, a 6-carbon sugar, is broken into two 3-carbon molecules of pyruvic acid. Pyruvic acid loses a carbon and becomes acetyl CoA. The acetyl CoA enters the Krebs cycle. Electrons from the Krebs cycle enter the electron transport chain, which produces ATP. The result is 34 molecules of ATP, which can be used as a source of energy for the life processes of cells.

Minor Nutrients and Metabolism

29

All multicelled organisms must carry on aerobic respiration because they cannot survive on the small amount of energy produced by anaerobic processes. Cellular respiration most commonly involves the breakdown of glucose for ATP production using three connected chemical pathways: glycolysis, the Krebs cycle, and the electron transport chain (Figure 3.2). Glycolysis is sometimes an anaerobic process—the reactions can continue in the absence of oxygen. Other times glycolysis, as well as the Krebs cycle and the electron transport chain, are aerobic processes in that they cannot continue in the absence of oxygen. Glucose enters glycolysis as a 6-carbon sugar and comes out as two 3-carbon molecules of pyruvic acid. This process also results in the production of two ATP molecules. Pyruvic acid loses a carbon dioxide and forms an acetyl group that combines with a form of vitamin B6, resulting in a compound called acetyl CoA. This compound enters the second phase of glucose oxidation, the Krebs cycle. Because two pyruvic acids produce two acetyl groups, the Krebs cycle runs twice

Table 3.2 Approximate Number By Activity activity

100 lb person

of

Calories Burned

150 lb person

per

Hour

200 lb person

Bicycling, 6 mph

160

240

312

Bicycling, 12 mph

270

410

534

Jogging, 5.5 mph

440

660

962

Jogging, 10 mph

850

1,280

1,664

Jumping rope

500

750

1,000

Swimming, 25 yds/min

185

275

358

Swimming, 50 yds/min

325

500

650

Walking, 2 mph

160

240

312

Walking, 4.5 mph

295

440

572

Tennis (singles)

265

400

535

30

DIGESTION AND NUTRITION

for each glycose molecule broken down. The pyruvic acid can also be used to form the amino acid alanine, which can be transformed into other amino acids. The acetyl group is added to an existing compound in the Krebs cycle, and the resulting series of chemical reactions results in the removal of electrons from hydrogen atoms to send to the third phase, the electron transport chain, and the production of a single ATP molecule through substrate phosphorylation. The final chemical in the series receives a new acetyl group and the cycle begins again. The waste product of the Krebs cycle is carbon dioxide. Several intermediate chemicals produced during the Krebs cycle can be removed for amino acid synthesis. These amino acids can also be fed into the Krebs cycle through these intermediate chemicals. The last pathway, the electron transport chain, is a series of chemical reactions that pass electrons from one chemical to the next. During this process, 34 ATP molecules can be produced for each glucose molecule that started the process. It is possible to make a total of 38 ATP molecules through the three pathways. Because many of the intermediate compounds are used for other purposes, the maximum number of ATP molecules is seldom produced, except in skeletal muscle, where all of the ATP is needed for muscle contraction.

Connections Humans need to eat to gain energy for chemical reactions involving a type of chemical bond called a covalent bond. This bond keeps complex biological chemicals together and requires energy that is crucial for repair, growth, or development. Metabolic pathways for carbohydrates, proteins, and lipids intersect. Thus, the body can use nutrients to make needed compounds, or to convert the nutrients to compounds that can

Minor Nutrients and Metabolism

be burned for energy. Carbohydrates exist as mono­saccharides, disaccharides, and polysaccharides, depending on the number of sugar units. Monosaccharides include glucose, fructose, and galactose. Disaccharides include sucrose, lactose, and maltose. Biologically important polysaccharides come either from plants as starch or from animals as glycogen. Proteins are made from a mixture of 20 amino acids and fulfill a variety of functions in the body. Cholesterol and triglycerides are important dietary lipids. Triglycerides are an important form of long-term energy storage and are made from excess carbohydrates. Both vitamins and minerals are important in metabolizing the major nutrients of carbohydrates, proteins, and lipids. Deficiencies of vitamins or minerals compromise cell metabolism.

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4 Digestion, Absorption, and Elimination Let’s get back to Amy and her lunch, which was discussed in Chapter 1. The question is: When Amy eats her hamburger, fries, and chocolate shake, how do the nutrients in these foods get to the tissues in her body that need them? This may sound strange, but foods that are inside the digestive tract are not yet actually in the body. The digestive tract is a long tube (about 30 feet when relaxed) with openings at both ends (Figure 4.1). This tube is contained within the body, and anything that enters it must pass through the cells that make up the walls of the tube in order to enter the bloodstream and be transported to the body’s tissues. As food passes through the digestive tube, it is processed and broken down gradually so the nutrients (e.g., sugars, proteins, and fats) can be absorbed by the cells in the walls of the digestive tract. The hamburger bun, the fries, and the shake contain carbohydrates. Carbohydrates must be broken down to monosaccharides, or simple sugars. Glucose is the simple sugar used most in metabolism. The starch molecules in the bread and potatoes are made up of hundreds of monosaccharides. Other sugars, such as those in table sugar, milk, and beer, are disaccharides, and are made up of two monosaccharides 32

Digestion, Absorption, and Elimination

Figure 4.1 The digestive system is a tube within the body, with an opening at the mouth (for intake) and an opening at the anus (for excretion). The digestive system includes the mouth, esophagus, stomach, small and large intestines, and rectum.

bonded together. Anything larger than a mono­saccharide cannot be absorbed through the walls of the digestive tube, so any disaccharides not broken down to monosaccharides are used by bacteria living in the intestines. As a result of this

33

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DIGESTION AND NUTRITION

bacterial metabolism, some people experience abdominal cramping and diarrhea. These are symptoms that may occur when a person is lactose intolerant, which is discussed in Chapter 8. The meat in the burger is a good source of protein. Proteins are composed of hundreds of amino acids and must be broken down into individual amino acids in order to be absorbed into the cells lining the digestive tube. The body will use these amino acid building blocks to make body proteins. The beef of the hamburger also contains fats, as does the oil in which the fries are prepared. Fats, also called lipids, may or may not be broken down to get them into the cells lining the digestive tube. (Different types of fats were described in Chapter 2.) Cholesterol is absorbed whole, while triglycerides are broken apart every time they enter or leave a cell. (Triglycerides cannot pass through any cell membrane intact, but cholesterol can.) Triglycerides are composed of a single glycerol and three fatty acid chains. The fatty acid chains can be either saturated or unsaturated. Because the fatty acid chains are absorbed through the digestive tube “as is,” the body will build up a supply of triglycerides that contains whichever type of dietary fatty acids have been ingested. If a person eats food that is high in saturated fatty acids, for example, the saturated fatty acids will be transported to the tissue of the body and stored there.

Structure of the Digestive Tube Throughout the digestive tube, the walls of the organs are made up of four tissue layers: the mucosa, submucosa, muscularis, and serosa, or adventitia (Figure 4.2). The innermost layer of the digestive tube is the mucosa. This layer is composed of three parts: the epithelium, the lamina propria, and the muscularis mucosa. The innermost part of the mucosa is the epithelium. Most of the epithelial layer is made up of a single layer of cells called columnar epithelial cells. These

Digestion, Absorption, and Elimination

Figure 4.2 The walls of the digestive tube are made up of four layers: the mucosa, the submucosa, the muscularis (external muscle layer), and the serosa.

cells are lined up like columns with one end exposed to the material in the digestive tube and the other end forming the connection between the epithelial layer and the tissue beneath

35

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DIGESTION AND NUTRITION

the lining. Everything absorbed into the body must pass through these cells. In addition to the columnar cells, mucussecreting cells called goblet cells because of their unique shape (narrow bottom and wide top) are found throughout the tube. The mucus becomes especially important farther along in the tube, when the intestinal contents are dehydrated into feces. At the beginning of the digestive tube, the epithelium is made up of squamous epithelial cells, which are specialized for protection. These cells, which are flat and resemble a pancake with a nucleus in the center, can be stacked up, which helps protect the tissue underneath them. If a single layer of cells lined this part of the digestive tract and these cells were to die, the tissue within the wall of the tube would be exposed and subject to further damage and infections from ingested material. Strong chemicals that are ingested may also be harmful until they are neutralized in the stomach. This protective layer of cells is found in the upper part of the digestive tube, as well as on the body surface, to protect the tissue from abrasion and prevent further damage to the body. Beneath the epithelium is the lamina propria, a layer of connective tissue that supports the absorptive cells. This layer contains loose connective tissue with blood and lymphatic capillaries to carry material away from the columnar cells and to the body’s tissues. The muscularis mucosa has a thin layer of smooth muscle around the lamina propria. This layer helps move food through the digestive tube. The second major layer of the digestive tube wall is the submucosa. This layer, similar to the lamina propria but thicker, has connective tissue and blood vessels. The submucosa also has some nerves to assist in regulating the digestive process, lymph nodules to screen for foreign material that may cause antibodies to be made, and sometimes glands, depending on the part of the tube. These adaptations to the submucosa will be discussed in the following chapters.

Digestion, Absorption, and Elimination

The third layer of the digestive tube wall is the muscularis. This layer is similar to the muscularis mucosa, but is much thicker and has two layers of smooth muscle. The inner layer of muscle is arranged in a circular pattern around the tube. The outer layer of muscle cells runs parallel to the tube. Contraction of these muscle layers propels the contents through the digestive tube via a process called peristalsis (Figure 4.3). Contraction of the inner layer nudges the material along with constrictions of the rings of muscle. Contraction of the outer layer pushes digestive contents through the tube. The parallel arrangement causes waves of constriction that press on the tube, pushing the material. The muscularis has nerves between the two layers of smooth muscle that assist in regulating peristalsis. The last and outermost major layer of the digestive tube is called the serosa, or adventitia. On the esophagus, the outer covering is called the adventitia. At the end of the digestive tube, the covering is called the serosa. This covering is also called the visceral peritoneum, meaning the connective tissue covering of the visceral organs in the abdominal cavity. This layer is made of dense, fibrous connective tissue throughout the tube. The only difference is the name given to the covering, based on the location of that part of the tube. The serosa/ adventitia of both cattle and pigs is used commercially as the outer covering or casing on sausages, kielbasa, and certain types of hot dogs.

Summary of the Digestive Process The process of taking food into the mouth is called ingestion. In the mouth, the ingested food is broken down into smaller pieces and mixed with saliva. The resulting food mass, called a bolus , is sent to the pharynx and then into the esophagus. In addition to the physical breakdown of food into smaller pieces, some chemical digestion begins in the mouth, especially for

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DIGESTION AND NUTRITION

starches. The esophagus transports the bolus of food to the stomach. A detailed description of this part of the process can be found in Chapter 5. The stomach acts as a blender, mixing the food with digestive juices secreted by specialized cells in the stomach lining. One of the digestive chemicals produced in the stomach is hydrochloric acid at a concentration strong enough to eat away shoe leather. A large amount of mucus present in the stomach protects the cells of the stomach lining from this acid. The contents of the stomach are squirted into the small intestine at regular intervals. Locally produced hormones control this process. The digested food at this stage is called chyme. It consists of a combination of ingested food, saliva, and stomach juices. Additional digestive juices enter the small intestine from the pancreas and gallbladder. The pancreas contributes enzymes that break down what is left of starch, protein fragments, and triglycerides. Bile from the gallbladder aids in the absorption of fats in the small intestine. The final breakdown of the ingested food, including disaccharides, occurs at the surface of the columnar cells lining the tube. The nutrients are then absorbed into the lining cells. Nearly all the absorption of nutrients occurs in the small intestine. When nutrients leave the digestive tract, most enter the bloodstream and go either to the body’s tissues or to the liver. The liver is an accessory organ to the digestive tract that regulates much of what is removed from the blood. A specific description of this part of the process can be found in Chapter 5. Most of the water that enters the digestive tract with food or from the digestive juices of the stomach and pancreas is actively removed from the tube by the large intestine. The Figure 4.3 (opposite page) With swallowing, food leaves the mouth and enters the esophagus. It is pushed down the esophagus by waves of contraction called peristalsis. The walls of the esophagus alternately constrict and relax to move the bolus of food toward the stomach.

Digestion, Absorption, and Elimination

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DIGESTION AND NUTRITION

removal of most of the water from the digestive tube creates the material that is eliminated from the body in the form of feces. A large number of goblet cells located in this portion of the tube produce the mucus necessary to move the feces through the rest of the tract. Whatever has not been broken down or absorbed in the digestive process is eliminated through the rectum and the anus. This is discussed further in Chapter 6.

Connections Nutrients must be broken down to a size that allows them to be absorbed into a microscopic cell. Each type of nutrient has a basic building block that can be absorbed. For sugars, this basic unit is a monosaccharide. For proteins, this is an amino acid. Lipids in the form of triglycerides are broken into glycerol and fatty acid chains, while cholesterol is absorbed intact. The wall of the digestive tract is made up of four major layers: mucosa, submucosa, muscularis, and a connective tissue covering called a serosa or adventitia. Each section of the digestive tube has specific functions. The mouth and esophagus ingest and transport food, and the stomach blends it with digestive juices. The final breakdown of the food is completed in the small intestine where nutrients are absorbed. The large intestine salvages most of the water from the intestinal contents and prepares the solid waste for elimination.

5 Chewing and Swallowing Digestion starts when Amy takes a bite of her burger, chews, and swallows it. Chewing starts the breakdown of food to a manageable size that can pass into the cells lining the digestive tube. When Amy bites into her burger and bun, teeth called incisors cut it into bite-size pieces. Other teeth called canines, or eyeteeth, help her tear up some of the food. While Amy chews, teeth called premolars and molars grind the food. With the help of the tongue, the food, now a pulpy mass, is formed into a rounded lump, or a bolus. This bolus is a mixture of saliva and the ingested food. Her tongue moves the food around in her mouth, from side to side, and, eventually, toward the back of her mouth so she can swallow the bolus.

Mouth The mouth starts with the lips in front and extends back to an area called the oropharynx. The oropharynx is where both food and air pass to enter the digestive tube and respiratory tract, respectively. The lips have a circular skeletal muscle called the orbicularis oris, which allows lip movement, as in a pucker or a smile. The skin at the edges of the lips is thin, allowing the natural red coloring of blood to tint the lips. The 41

42

DIGESTION AND NUTRITION

Figure 5.1 Humans are born with two sets of teeth: (a) deciduous (milk) teeth and (b) permanent teeth. The deciduous teeth erupt from the gums around six months of age, and generally finish growing at about two years. At about age 7, children begin to lose their baby teeth, which are gradually replaced by the permanent teeth.

lips do not have any sweat glands, so they must be moistened on a regular basis or the skin may become dry, crack, and possibly bleed.

Chewing and Swallowing

The sides of the mouth, or cheeks, contain a skeletal muscle called the buccinator. This muscle helps move food around in the mouth and also helps in forming facial expressions. The inside of each cheek is covered with a stratified squamous epithelium that is not keratinized like skin, so it is not dry, but kept moist by saliva. This type of epithelium is discussed in Chapter 4. The top, or roof, of the mouth is called the palate. The front part of the roof of the mouth is the hard palate and has bone above the tissue lining the mouth. When a person chews and forms certain speech sounds, the tongue presses against the hard palate. The rear portion of the roof of the mouth is called the soft palate and has skeletal muscle, not bone, above the mouth lining. A small, fingerlike projection of the soft palate at the rear of the mouth is called the uvula. The soft palate rises during swallowing to block the opening to the nasal cavity at the top of the oropharynx, preventing a person from inhaling and swallowing at the same time.

Teeth Amy’s teeth, just as with all adults, are her second set of teeth. Humans are born with two sets of teeth hidden in the upper and lower jaws. At about six months of age, children begin to get their first teeth, called baby teeth, or deciduous teeth. The first set of teeth continues to penetrate through the gums up to about two years of age, until all 20 teeth have emerged. The first set of teeth consists of 8 incisors, 4 canines, and 8 molars (Figure 5.1a). The second set of teeth, called permanent teeth, begins to push the baby teeth out when a child is about 7 years of age. The permanent teeth continue to emerge up to about age 25, when the wisdom teeth have emerged. A complete set of permanent teeth consists of 8 incisors, 4 canines, 8 premolars, and 12 molars, totaling 32 teeth (Figure 5.1b). All teeth have basically the same structure (Figure 5.2). The visible part of the tooth, the crown, is covered with hard enamel. The enamel cannot be replaced. If it is damaged, the

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DIGESTION AND NUTRITION

Figure 5.2 Teeth are made up of two sections: the crown, which lies above the gum, and the root, which lies below the gum. The crown covers the dentin, which reaches into the root cavity. The root contains blood vessels and nerves.

underlying tissues of the tooth may be exposed, resulting in the degeneration and loss of the entire tooth. The shape of the crown determines whether the tooth is an incisor, canine, or molar. The neck of the tooth starts at the gum line and extends through the gum. More of this portion of the tooth may be

Chewing and Swallowing

exposed as the gum recedes with age. Below the neck, the root of the tooth is embedded in the jawbone. The root is held in place by a calcified type of connective tissue called cementum that attaches the tooth to a periodontal ligament. Depending on the type of tooth, teeth may have from one to three roots. Dentin, a bonelike substance, extends through the crown, neck, and root of the tooth and makes up the bulk of the tooth. Although enamel cannot be replaced, dentin can, so some repair of damage or decay takes place throughout a person's lifetime. The central core of each tooth is filled with pulp, a mixture of connective tissue, blood vessels, and nerves. This pulp extends through the tooth to the base and forms the root canal.

Tongue The tongue moves the food around in the mouth, forming the bolus of food and saliva. The tongue is also involved in speech. Some of the skeletal muscles in the tongue are used to change its shape. These muscles are not attached to bones, but are contained completely within the tongue. Other muscles, which are attached to the bones of the skull and in the neck, are used to change the tongue’s location. The surface of the tongue has projections of tissue called papillae. Some of the papillae contain taste buds, which are sensitive to tastes of sweet, sour, salty, and bitter. At the back of the mouth, in the oropharynx, individuals perceive a fifth taste that is stimulated by an amino acid called glutamate, found in some foods as well as in the seasoning monosodium glutamate, or MSG. Tonsils are found on the rearmost area of the tongue and in the surrounding soft tissue of the oropharynx. The tonsils are aggregates of lymphoid tissue that screen the incoming food for microorganisms that might attack the body. If these organisms are detected, the lymphocytes in the tonsils make antibodies that help defend the body from attack. Some bacteria, such as Streptococcus, can hide in the tonsils and cause

45

4

dIgestIon and nutrItIon

recurring throat infections. If the infection is not treated properly, these bacteria continue to hide in the tonsils and cause infection again.

SALivA As stated earlier, digestion is a process that breaks down food to sizes that can be absorbed by cells. There are two components to the process: physical and chemical. The physical aspects of digestion in the mouth occur when we bite, tear, and chew food. This breaks the food into smaller, but still fairly large, chunks. Chemical digestion in the mouth involves saliva. Most of the saliva is secreted by three pairs of salivary glands: the parotid, submandibular, and sublingual. These glands are found under the tongue and in front of the masseter muscle , the muscle that provides most of the force in biting. The parotid glands produce a watery secretion, the sublingual glands produce a very mucoid secretion, and the submandibular glands make a combination of a watery and mucoid secretion. The combination of secretions from these

YOUR HEALTH: THE IMPORTANCE OF BRUSHING YOUR TEETH tooth decay begins when dental plaque—a layer of bacteria, trapped sugars, and mouth debris—sticks to teeth. Plaque provides a safe haven for bacteria to live and metabolize the bits of trapped food. Bacterial waste consists of assorted acids that damage tooth enamel. once the enamel is damaged, the bacteria break down the proteins of the tooth and cause tooth decay or cavities. Periodontal disease may result if the plaque builds up on the gums. this buildup, called tartar or calculus, may damage the seal between the tooth and the gums, allowing bacteria to get into the gums and cause serious infections. these infections can be quite painful, and may sometimes lead to loss of the affected tooth or teeth. In extreme cases, they may also be life-threatening.

Chewing and swallowing

glands makes up normal saliva. If a person is dehydrated, the saliva produced is thick and comes mostly from the sublingual glands. Saliva has several purposes, including cleansing the mouth, dissolving food to enhance taste, moistening food to form a bolus, and starting the chemical digestion of starches and some lipids. Two enzymes in the saliva begin the chemical breakdown of food: amylase and lingual lipase. Amylase starts the digestion of starch by breaking down the complex structure of starch molecules into smaller combinations of glucose units that are separated further along the digestive tube. Because amylase works at an alkaline ph, saliva contains bicarbonate to maintain these conditions. Lingual lipase is a form of lipase that separates triglycerides into their components of glycerol and fatty acids. A person produces about 1 to 1.5 quarts (about 1 to 1.5 liters) of saliva per day. The salivary glands produce saliva when stimulated by the presence of food in the mouth or by certain acidic foods, such as vinegar or citric juices. Sometimes, the thought or smell of food will result in saliva release. In addition, if the small or large intestine is irritated by some substance, such as excess acids or bacterial toxins, the salivary glands will release saliva.

phAryNx The oropharynx, at the back of the mouth, is just one of three parts of the pharynx . The area above the oropharynx, the nasopharynx , is exclusively part of the respiratory tract. The area immediately below the oropharynx, the laryngopharynx , serves as a passage for both air and food. The oropharynx and the laryngopharynx are lined with stratified squamous epithelial cells to protect the underlying tissue from damage. This epithelium has the first goblet cells found in the digestive tube. The goblet cells secrete mucus that helps the bolus of food get to the esophagus and stomach. The muscularis

4

48

DIGESTION AND NUTRITION

layer of the pharynx has two layers of smooth muscle, but in the opposite arrangement from that found throughout the rest of the digestive tube. Here, the inner layer is longitudinal and the outer layer is circular. The two layers work together to propel food by peristalsis to the stomach. Figure 5.3 illustrates the anatomy of both the pharynx and the esophagus.

Figure 5.3 Structures of the pharynx and esophagus. The esophagus connects the pharynx to the stomach.

Chewing and Swallowing

Esophagus Once the bolus of food passes through the oro- and laryngopharynx, it enters the esophagus. This muscular tube, located behind the windpipe, is collapsed when empty. The esophagus is about 10 inches (25.4 cm) long, starts at the bottom of the laryngopharynx, and ends at the opening of the stomach, which is called the cardiac sphincter. (A sphincter is a circular muscle, usually made up of smooth muscle cells, that opens and closes a tube.) The cardiac sphincter loosely controls the opening of the esophagus into the stomach. The esophagus passes through the diaphragm located just above the stomach. This muscle assists the cardiac sphincter in limiting access to the stomach. The esophagus is made up of four layers of tissues. The epithelial lining, the innermost layer, is made up of stratified squamous cells, as in the mouth and pharynx. Next, the mucosal and submucosal layers of the esophagus fold along the length of the tube when it is empty. The muscularis has two layers of muscle, the inner circular and the outer longitudinal, but not all of the muscle is the same type. The first third of the esophagus has skeletal muscle in the muscularis layer, the last third has smooth muscle, and the middle portion has a mixture that gradually goes from skeletal to smooth muscle cells. Finally, the esophagus has an outer layer called the adventitia that blends the tube covering with the surrounding tissue and holds the esophagus in place in the throat.

Swallowing Swallowing is actually a complex process. When a person swallows, a series of reflexes occur that ensure that the action occurs properly. First, the soft palate rises to close the connection with the nasal passage. Then, the tongue blocks the possibility of reentry of the food into the mouth. The larynx rises, causing a lid-shaped piece of cartilage and soft tissue called the epiglottis to cover the opening to the trachea, closing off the windpipe. The presence of the food in the esopha-

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dIgestIon and nutrItIon

YOUR HEALTH: HEARTBURN heartburn occurs when the stomach contents pass back up into the esophagus. this regurgitation can occur when a person vomits, the stomach is overfull, or a person is obese, pregnant, or running. the cardiac sphincter and the diaphragm do not entirely close off the connection to the stomach. It is fairly easy to overcome these barriers and bring stomach contents back up into the tube. Because the pH of the stomach fluids is usually below 4, or about the strength of a car’s battery acid, the acid burns the epithelial layers of the esophagus and may cause scarring of the tissue. If this occurs, the esophagus does not fold when empty or propel food properly through peristaltic contractions. the scarring also leaves the tissue susceptible to further damage due to the loss of the stratified epithelial layers that protect the underlying tissue.

gus stretches the tube and causes the muscle contractions that result in peristalsis. Solid food passes to the stomach in 4 to 8 seconds, liquids in 1 to 2 seconds. When food is swallowed, the bolus no longer resembles the food it started out as. The nutrients have been partly digested, but they are still not ready to be absorbed into the body. Further digestion is necessary and continues in the stomach and small intestine, as will be described in the next chapter.

ConneCtions Digestion starts with the first bite of food. Teeth cut, tear, and grind food and saliva into a paste. Baby teeth start erupting from the gums at about 6 months of age and begin to be replaced by permanent teeth at about 7 years of age and continue to age 25. All teeth have a common structure and have hard enamel to protect underlying tissues from decay and damage.

Chewing and swallowing

The mouth is bordered by the lips, cheeks, palate, floor, and pharynx. The tongue helps to manipulate food during chewing, and it is also essential in speech. Taste buds on the tongue are stimulated by different chemicals. The saliva produced by salivary glands moistens the food and begins the chemical digestion of starches and triglycerides. Food, in the form of a bolus, passes from the mouth to the pharynx and into the esophagus for passage to the stomach. The process of swallowing is a reflex that propels food and prevents it from entering the respiratory tract.

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6 The Stomach and Small Intestine The bolus of food that enters the stomach contains some partially digested food and some food that has not been broken down at all. Amylase and lingual lipase begin the digestion of starch and triglycerides, respectively. Digestion of protein begins in the stomach. Before they leave the small intestine, all of the nutrients that can be absorbed into the lining cells of the digestive tube will be absorbed.

Stomach The connection between the stomach and the esophagus is called the cardiac sphincter. Below the esophagus, the stomach works to digest proteins in the food (Figure 6.1) and turns the bolus of food into chyme. The muscularis in the stomach consists of three distinct layers. The positioning of these layers allows the stomach to constrict in all directions. In the innermost layer, the fibers are oblique to the other layers The middle layer is arranged in a circular pattern, making it look similar to a donut. The outermost layer runs longitudinally. The stomach, which is really a section of the digestive tube, is shaped somewhat like the letter J. The largest part of 52

The Stomach and Small Intestine

Figure 6.1 The stomach is located below the diaphragm. It connects to the esophagus at the cardiac sphincter and to the duodenum of the small intestine at the pyloric sphincter. Three layers of smooth muscle make up most of the wall of the stomach. Folds of the mucosa called rugae increase the surface area of the organ.

the stomach, the body, is where most of the stomach’s digestive activity occurs. The portion of the stomach above the body but below the connection to the esophagus is called the

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fundus. When food enters the stomach, some of it stays in the

fundus while the rest is mixed with stomach fluids. While the food is in the fundus, salivary amylase continues to break up starch. As the food in the body of the stomach leaves to enter the small intestine, more food is brought from the fundus to continue the digestive process. In this way, the fundus acts like a storeroom for excess food until there is space in the body of the stomach. An empty stomach is about the size of a fist, while a full one is considerably larger, especially after a large meal. When the stomach is empty, large folds called rugae are created in the mucosa that can be seen without magnification. These folds expand the surface area of the inside of the stomach. The mucosa of the stomach has several adaptations not found anywhere else in the digestive tract. The epithelial cells extend into the underlying layers of the mucosa to form depressions called gastric pits (Figure 6.2). These pits are lined with a mixture of columnar epithelial cells and special cells that secrete chemicals to aid in digestion. Goblet cells secrete mucus to protect the stomach lining from other secretions, especially the hydrochloric acid secreted by the parietal cells. The acid aids digestion by indiscriminately breaking up larger compounds into smaller ones. The acid digests everything, including bacteria and medications. Parietal cells also secrete a chemical called intrinsic factor that is necessary for the absorption of vitamin B12 in the small intestine. If intrinsic factor is not available, the vitamin is not absorbed and a syndrome called pernicious anemia results. A third type of cell found in the epithelia of the gastric pits is the chief cell. This cell secretes a chemical called pepsinogen. When pepsinogen comes in contact with hydrochloric acid, it is converted into an active enzyme called pepsin. Pepsin begins the digestion of proteins by breaking large, complex proteins into smaller molecules that will be further

The Stomach and Small Intestine

Figure 6.2 Gastric pits, found in the lining of the stomach, contain special cells that secrete chemicals used in digestion. A micrograph of gastric pits shows the cells up close.

broken down in the small intestine. If hydrochloric acid is not present, pepsin will not be formed from pepsinogen and the digestion of protein will not begin. The fourth specialized epithelial cell, called the G cell, secretes a hormone called gastrin that is primarily responsible

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for stimulating the other three types of cells. Stomach fluids are produced when the G cells are active.

Regulating Digestion in the Stomach Digestion in the stomach can be stimulated in a variety of ways. The thought, sight, or smell of food can stimulate the stomach to secrete digestive fluids. When food enters the stomach, it stretches the walls of the stomach, resulting in pressure on stretch receptors , specialized nerves within the smooth muscle layers of the stomach. These nerves relay a signal that stimulates gastrin secretion. In addition to the action of the stretch receptors, certain chemicals, such as caffeine and alcohol, cause stomach fluids to be produced. Highly alkaline food also causes stomach secretions to flow. Increasing amounts of hydrochloric acid in the stomach cause the parietal cells to decrease secretion of the acid, slowing production of pepsin and slowing down the initial breakdown of proteins. Stomach digestion is also slowed by the actions of the small intestine. When chyme, the acidic stomach contents, enters the small intestine, the nervous system stimulation of the G cells is inhibited. The presence in the small intestine of protein fragments called peptides and fatty acids from triglyceride breakdown also inhibits the nervous system's stimulation of the G cells and slows the mixing of the stomach contents. The interaction of the stomach and small intestine ensures that the small intestine receives chyme in amounts that it can handle. In addition to continuing digestion in the intestine, this control process includes the neutralization of stomach acid. The small intestine does not have the relatively thick protective layer of mucus found in the stomach. The hydrochloric acid would harm the intestinal lining and cause a type of ulcer (see the “Your Health” section concerning ulcers on page 58). Few materials are absorbed through the stomach lining. The stomach functions to prepare food for further digestion and absorption in the small intestine. Glucose, caffeine, and

The Stomach and Small Intestine

alcohol are three chemicals that are absorbed through the stomach lining. Water is absorbed in the stomach only if it contains a sufficient amount of dissolved glucose. The formulation of sports drinks for athletes takes advantage of this dual absorption to rapidly increase the body’s energy supply for muscle metabolism and to rehydrate the body after extensive sweating. The stomach empties slowly, about one ounce of fluid at a time, into the small intestine. It may take hours to empty the stomach after a big meal. At the junction of the stomach and the small intestine, the pyloric sphincter regulates the amount of food that enters the small intestine. Unlike the cardiac sphincter, the pyloric sphincter cannot be easily forced open. Its opening and closing is controlled by the amount of food in the stomach and by feedback from the small intestine.

Small Intestine In the small intestine, digestion is completed. The products of digestion are absorbed through the digestive tube lining and transported to the rest of the body’s tissues by the blood and lymphatic vascular systems. The small intestine has three sections: the duodenum, the jejunum, and the ileum. Chyme from the stomach enters the duodenum portion of the small intestine, which is about 10 inches long. The jejunum, or middle segment of the small intestine, is about 3 feet long. The last portion of the small intestine, the ileum, is about 6 feet long. The small intestine is bent, folded, and twisted to fit into the abdominal cavity. The adaptations of the digestive tube wall in the small intestine involve the mucosa and submucosa. The mucosa is folded into circular folds that increase the surface area of the small intestine and force the intestinal contents to go through the tube in a spiral pattern. Both of these effects increase the contact of the tube’s contents with the epithelial layer, thereby

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increasing the chances of nutrients being absorbed into the lining cells.

YOUR HEALTH: ULCERS Imagine the body attacking and damaging its own tissues. When a person has a gastric or peptic ulcer, the hydrochloric acid attacks the walls of the stomach, damaging the mucosa. this may lead to bleeding so severe that it compromises the body’s ability to deliver adequate oxygen to tissues. such bleeding results in severe anemia. until the 10s, ulcers were believed to be caused by stress, alcohol use, or use of excessive amounts of aspirin. aspirin and other nonsteroidal anti-inflammatory drugs, such as ibuprofen, are still believed to be a cause of ulcers, but they are not the most common cause. a bacterium called Helicobacter pylori is now believed to be the primary cause of ulcers and the cause of their recurrence. H. pylori burrows under the mucus layer in the stomach and produces ammonia, which neutralizes stomach acid in the small area surrounding the bacteria. the microorganism also makes enzymes that damage the mucosa and allow the hydrochloric acid to further damage the stomach lining in areas where ammonia is not present. this damage to the mucosa may go through the lining to the blood vessels in the submucosa and result in significant bleeding in the digestive tract. If the damage is severe, a hole may be created in the wall of the stomach that would allow food and microorganisms access to the abdominal cavity, resulting in a lifethreatening infection called peritonitis. Before the bacterium was identified as the cause of ulcers, 5% of ulcers recurred. treatment usually involved removing the damaged part of the stomach. Currently, ulcers are treated by giving the person a 10- to 14-day supply of two antibiotics and a bismuth compound to enhance healing of the stomach “sore.” the antibiotics kill the bacteria while the bismuth salicylate protects the stomach lining from acid and inhibits growth of the bacteria.

the stomach and small Intestine

The mucosa also has fingerlike projections called villi, which greatly increase the surface area for absorption. The center of each villus contains two capillaries: a blood capillary and a lymphatic capillary. The blood capillary contains blood and connects an artery to a vein. The lymphatic capillary contains a fluid called lymph that will pass into the lymphatic system, and ultimately into the blood. Materials that have been absorbed into the digestive tube’s lining pass into one of these capillaries. Water-soluble materials, such as sugars and amino acids, enter the blood capillary, while fat-soluble material, such as cholesterol and triglycerides, enter the lymphatic capillary. All blood drained away from the digestive tract goes directly to the liver. As will be discussed in Chapter 6, the liver is the body’s chemical processing plant and uses the nutrients from digestion for multiple purposes. The fats in the lymphatic capillaries do not go directly to the liver, but are dumped into the bloodstream by way of the jugular and subclavian veins in the neck. These fats are sent around to the body and are used by the body’s tissues as a source of energy or as structural materials needed for growth or repair of body tissues. The membrane of each mucosal epithelial cell has projections from the surface called microvilli (Figure 6.3). These projections are a third way of increasing the surface area of the small intestine to aid in the digestion and absorption of digested material. The adaptations in the submucosa help to identify the three portions of the small intestine. The duodenum has glands called brunner’s glands in the submucosa. The Brunner’s glands secrete an alkaline mucus that helps to neutralize the acidic chyme from the stomach. The ileum has clumps of lymphoid tissue called peyer’s patches in its submucosa. This lymphoid tissue helps to screen the ingested material in a way similar to what the tonsils do. The jejunum has no specialized microscopic adaptations in its submucosa.

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The Stomach and Small Intestine

Accessory Organs: The Liver, Gallbladder, and Pancreas The liver and the gallbladder are two important accessory digestive organs that work with the small intestine. The liver has several important functions in the body. It regulates carbohydrate, protein, and lipid metabolism and detoxifies body wastes and drugs that have entered the body. In addition, the liver eliminates bilirubin , a waste product of dead red blood cells, by incorporating it into bile. This fluid, which is stored in the gallbladder, helps digestion by emulsifying fats into smaller molecules for absorption. Bile is composed of bile salts , cholesterol, and phospholipids, as well as other substances. When needed, bile is released from the gallbladder into ducts that connect to the small intestine. The pancreas produces hormones, digestive enzymes, and bicarbonate to deliver to the duodenum to help digestion. Insulin and glucagon are two pancreatic hormones that work within the body to control blood glucose levels.

Digestion in the Small Intestine When acidic chyme enters the duodenum, it triggers several events. The acid, along with protein fragments and fatty acids in the chyme, causes cells at the beginning of the duodenum to secrete intestinal fluid. About 1 to 2 quarts (1 to 2 liters) of this digestive fluid is produced each day. The intestinal juice, which contains some mucus, is alkaline and helps to neutralize the acidic chyme and protect the duo­denum from the effects of the acid. The chyme also causes the release of two hormones from the duodenum. The hormone cholecystokinin (CCK) causes the gallbladder to constrict and pump Figure 6.3 (opposite page) Villi and microvilli act to increase the surface area of the small intestine, thus increasing the potential for nutrient absorption. Villi are fingerlike projections on the surface of the intestine, and microvilli are smaller projections stemming from the villi.

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bile into the small intestine via the bile duct. CCK also causes the pancreas to secrete digestive enzymes into the duodenum. The hormone secretin causes the pancreas to secrete large amounts of bicarbonate into the small intestine. The bicarbonate neutralizes most of the hydrochloric acid from the stomach. After the neutralization occurs, the small intestine contents are alkaline, creating the conditions needed for the digestive enzymes to work. Digestive enzymes break starch, proteins, triglycerides, and nucleic acids into intermediate-sized molecules. Pancreatic amylase breaks down starch into smaller pieces, including the disaccharide maltose, but not into glucose molecules. Thus far in the digestive process, the disaccharides, such as sucrose (from table sugar and fruits) and lactose (from milk) that are ingested, have not been broken down. Proteins are broken into peptides, which are short chains of amino acids, by several pancreatic enzymes that are collectively called peptidases. These include trypsin, chymotrypsin, elastase, and carboxypeptidase. The peptides that are formed as a result of the action of these enzymes are structurally very short and will be further broken down at the surface of villi and microvilli. Dietary triglycerides are broken apart by pancreatic lipase. The nucleic acids, DNA and RNA, are broken into nucleotides (their building blocks) by enzymes called nucleases. The nucleotides will also be further digested at the surface of the villi. The final digestion of material occurs at the villi. Enzymes from the epithelial cells of the villi break down disaccharides, peptides, and nucleotides into their most basic components,

Figure 6.4 (opposite page) Because fats are not water soluble, they cannot easily diffuse across cell membranes. Thus, the body has found an alternate way to transport fat droplets into cells. First, the lipids combine with bile salts to form emulsion droplets. These droplets are then digested by enzymes called lipases and eventually form micelles, which can pass into the cell.

The Stomach and Small Intestine

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which makes them suitable for absorption. This process works very efficiently because the final breakdown of nutrients occurs at the exact site of nutrient absorption. This material passes through the lining cell membranes and into the blood capillaries of the villi almost immediately. Lipid absorption is a more complicated process. Glycerol and short-chain fatty acids from triglycerides are absorbed by simple diffusion across the cell membranes in the digestive lining. Cholesterol and long-chain fatty acids cannot diffuse through the cell membranes and must be handled differently. Bile salts combine with these fats to form tiny spheres called micelles (Figure 6.4). These spheres can pass through the cell membranes and transport the fats into the cells. By the time the fats leave the lining cells, they have been combined with cell proteins that help carry them in the lymph and blood. This combination of cholesterol, triglycerides, and lining cell proteins is called a chylomicron. The chylomicrons will circulate in

Table 6.1 Summary

of the

Digestive Process

organ

source: enzymes/chemical action

Mouth

Saliva: Amylase

Digests starch

Saliva: Lingual lipase

Digests triglycerides

Goblet cells: Mucous

Protects stomach lining

Parietal cells: Hydrochloric acid

Converts pepsinogen to pepsin: As pepsin, digests proteins

Stomach

Chief cells: Pepsinogen Gallbladder

Bile salts

Emulsifies fats

Pancreas

Amylase

Digests starch

Lipase

Digests triglycerides

Nucleases

Digests nucleic acids

Peptidases

Digests proteins

Assorted enzymes

Final steps in digestion

Villi

The Stomach and Small Intestine

the blood, carrying triglycerides to body tissues. Eventually, the chylomicron remnants will be picked up by the liver. In addition to the nutrients discussed, other materials are also absorbed through the lining of the small intestine, including the electrolytes sodium, potassium, chloride, and bicarbonate and trace elements such as iodate, nitrate, calcium, iron, magnesium, and phosphates. About 9 quarts (8.5 liters) of water enter the small intestine each day. Some of this water is ingested with food (like Amy’s chocolate shake), but most of it enters into the system at various points during the digestive process to aid in the breakdown of food. The source of these fluids is approximately as follows: 1 to 2 quarts with food, 1 quart of saliva, 2 quarts of stomach (gastric) juices, 1 quart of bile, 2 quarts of pancreatic juice, and 1 quart of intestinal juice. Most of the water is reabsorbed through the small intestinal walls. Any leftover nutrients, waste, and some water leave the small intestine and enter the large intestine. Much of the water that enters the large intestine is reabsorbed into the blood so that very little is excreted in the feces. Table 6.1 summarizes the process of digestion.

Connections A bolus of food and saliva enters the stomach and is combined with digestive juices that are a mixture of strong acid and enzymes that begin the breakdown of proteins and lipids. The digestion of starch is halted by the hydrochloric acid in the stomach. The three layers of smooth muscle in the muscularis allow the stomach to compress its contents in all directions, blending the nutrients with digestive juices. The lining of the stomach contains specialized cells that secrete digestive enzyme precursors, acid, and a chemical to aid in vitamin absorption. Digestive functions in the stomach are controlled by hormones secreted by the lining cells and by hormones

(continues)

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(continued) originating in the first part of the small intestine. Few materials are absorbed through the stomach lining. The small intestine is divided into three parts: the duodenum, jejunum, and ileum. Structures found in the submucosa differentiate the three portions of the small intestine. Digestive juices are secreted into the duodenum from the gallbladder and pancreas. These juices contain bicarbonate to neutralize stomach acid; enzymes to break down carbohydrates; proteins and lipids; and bile to assist in lipid absorption. The mucosa of the small intestine has villi that increase the surface area of the tube and provide the enzymes necessary for the final steps of digestion.

7 The Large Intestine and Elimination The chyme that enters the large intestine is different from the chyme that enters the small intestine. All of the nutrients that can be absorbed from the food have been absorbed, but a type of digestion still takes place in the large intestine. Large numbers of bacteria that live in the large intestine finish digesting the chyme and use the nutrients for their own metabolism and growth. These bacteria make vitamins that are absorbed and used by the body. Vitamin K, which is one of those vitamins, is used by the liver to make proteins that play a role in blood clotting. Vitamin K is so important to us that if these bacteria are removed by diarrhea or excessive antibiotic use, various parts of the body can bleed two to three days after the bacteria are removed. The bacteria also make a variety of B vitamins that are used in the metabolism. Most of the 9 quarts (8.5 liters) of water that were present in the small intestine have also been removed by this point. About 1 quart (1 liter) is left and all but about 3 to 4 ounces (90 to 120 milliliters) of this water will be absorbed by the large intestine, producing the feces that will be eliminated from the body. 67

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Large Intestine Anatomy and Digestion The large intestine (Figure 7.1) is a tube about 5 feet (1.5 meters) long and 2.5 inches (6.4 centimeters) in diameter. It is wider than the previous sections of the digestive tube and can store material for 12 to 24 hours until elimination. The large intestine is connected to the small intestine by the ileocecal valve. There are four major sections of the large intestine: the cecum, colon, rectum, and anal canal. The cecum, which connects to the small intestine, is a 6-inch-long (15 cm), pouch-shaped portion of the tube that stores small amounts of chyme until it passes into the colon, the longest part of the large intestine. The appendix, which is attached to the cecum, is about 3 inches (7.6 cm) long. It is open to the cecum at one end and closed at the other end. The appendix has no function in digestion or absorption, but has several large aggregates of lymphoid tissue and may play a role in the immune system. The appendix can easily be twisted or blocked, causing an inflammation called appendicitis. If the appendix bursts, the bacteria that inhabit the large intestine can gain access to the abdominal cavity, causing acute infections that are difficult to treat and may lead to death. The second, and largest, section of the large intestine is the colon. This section is divided into four regions based on the direction or shape of the tube. After chyme leaves the cecum, it goes into the ascending colon, which is on the right side of the abdomen. The part of the colon that is in front of the stomach just under the diaphragm is called the transverse colon. On the left side of the abdomen is the descending colon. The colon begins to twist and bend down toward the middle of the body at this point. This portion is called the sigmoid colon and the twisting brings the tube in line with the last two, shorter, parts of the large intestine, the rectum and the anal canal.

The Large Intestine and Elimination

Figure 7.1 The large intestine consists of the cecum, colon, rectum, and anal canal. It is the last place where the body will remove nutrients before the waste is excreted.

There are several differences between the walls of the small and large intestines. First, the mucosa of the large intestine has no villi. The mucosal epithelium is made up of columnar cells and goblet cells. The mucus-secreting cells increase in number throughout the large intestine, which, in

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turn, increases the amount of mucus secreted. The mucus assists the passage of intestinal contents that are becoming increasingly dehydrated. The muscularis contains two layers of smooth muscle, but the outer layer is made up of three bands of muscle called the teniae coli. The regular constrictions of the teniae coli result in a muscle tone that gives the appearance of a series of pouches, or haustra, along the colon. The large intestine appears as though a piece of string were loosely tied around the diameter of the tube every couple of inches. As chyme travels through the large intestine, it moves from haustra to haustra with some mixing and storage at each stop. This process is called haustral churning and results in the physical digestion that occurs in the large intestine. As stated above, the chemical digestion that occurs in the large intestine is carried out by bacteria. These microorganisms are collectively called normal flora because they are normally found in the colon. E. coli, a bacterium that has received a lot of attention, is one of the organisms that normally exist in the large intestine. (It does not, however, normally exist in the urinary tract, where it is the most frequent cause of urinary tract infections.) The end of the intestinal tract is the anus, which is not far from the urethra, the opening for voiding urine. The transfer of bacteria from one opening to the other can result in a painful urinary tract infection. The intestinal bacteria use whatever carbohydrates have not been absorbed in the small intestine. The bacteria produce waste material that includes gases such as hydrogen, nitrogen, methane, carbon hydroxide, and dimethyl sulfide. About 1 pint (0.5 liter) of this gas is produced daily. The last component, dimethyl sulfide, is responsible for the strong odor of these gases. Some acids will also be produced and, in higher than normal concentrations, may cause abdominal pain and increase motility, or moveability of materials, in the large intestine, resulting in diarrhea.

The Large Intestine and Elimination

The bacteria also convert any amino acids that enter the large intestine into fatty acids and pass them in the feces. The bacteria convert bilirubin from the liver into a chemical called stercobilin, which produces the normal brown color of feces. If there is no bile coming from the liver or gallbladder, the feces are white or gray colored, and usually high in undigested fats because bile was not present to emulsify digestive fats. This whitish coloring and increased fat content indicates gallbladder problems. A person can live without the large intestine. In cases of cancer of the digestive tract, the large intestine can be removed and the person’s intestine attached to an opening in the abdominal wall. Feces are collected in a bag attached to the outside of the abdomen. This procedure is called a colostomy. Care must be given to supply the vitamins that are usually absorbed from the large intestine.

Defecation Defecation is the process of eliminating feces. Feces enter

the rectum, which is about 8 inches (20 cm) long, and travel to the anal canal. The anal canal is short, about 1.5 inches (4 cm) long, and ends at the body opening called the anus. There are two muscle sphincters in the anal canal, one at the beginning and one at the end. They are both involved in the elimination of feces. By the time the content of the digestive tube has reached the anal canal, it is made up of 2 to 3 ounces (60 to 90 milliliters) of water, a mixture of inorganic salts, dead epithelial cells, bacteria (about 30% by weight), unabsorbed material, and undigested material. Defecation results from the movement of chyme/feces through the large intestine by mass peristaltic movements during the day, building up feces in the rectum. The feces cause the rectal wall to stretch, resulting in initiation of the defecation reflex. This reflex involves local nerves and the two sphincters of the anal canal. The sphincter at the begin-

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ning of the canal is made of smooth muscle. The defecation reflex causes this sphincter to open, allowing feces to enter the anal canal. The second sphincter, made of skeletal muscle, is under voluntary control and can remain closed. At this point, local nerves send a signal to the brain of the need to have a bowel movement. In the case of an infant human, the signal is not received in time to voluntarily keep the second sphincter closed. When children become “potty trained,” they are able to keep this sphincter closed until they choose to defecate. A person who has become incontinent has lost the ability to keep the second sphincter closed.

DiArrheA Diarrhea, the frequent elimination of fluid feces, is a condition that occasionally happens to all of us. It can occur for a variety of reasons. Osmotic diarrhea occurs when a large

DID YOU KNOW? Fiber is important for digestion at any age, from teen to elderly. there are two types of fiber: soluble and insoluble. soluble types of fiber are found in beans, oat, barley, broccoli, prunes, apples, and citrus fruits. this fiber forms a gel that slows the movement of chyme through the intestine and binds cholesterol. ordinarily, two-thirds of the cholesterol that enters the intestine is reabsorbed. Fiber keeps cholesterol in the digestive tube until elimination, which helps to decrease the cholesterol in the body and lower the risk of heart disease and stroke. Insoluble fiber speeds up motility in the intestine and helps people have regular bowel movements. this type of fiber is found in fruit skins and the bran of wheat and corn. Increased fiber in the diet also decreases the risk of developing obesity, diabetes mellitus, atherosclerosis, hemorrhoids, and colorectal cancer.

the Large Intestine and elimination

amount of unabsorbed material is retained in the intestine. To keep this material in solution, more water than usual is retained in the tube. Medications containing large amounts of sodium and magnesium sulfates will have this effect. Diarrhea can also occur when the mucosa becomes inflamed or damaged. Inflammation of the digestive tube can result in large amounts of mucus and blood collecting in the intestine. The type of Salmonella bacteria that causes typhoid fever invades the intestinal wall, both damaging the wall and causing severe inflammation. The reabsorption of water that occurs in the intestines can be reversed by bacterial toxins, resulting in water pouring into the intestines, instead of being removed. Types of E. coli, Staphylococcus, and a related organism called Shigella cause diarrhea by this method. The microorganism that causes cholera produces a toxin that results in so much diarrhea that the person may die as a result of fluid loss. Peristalsis can be increased to the extent that the chyme is forced through and out of the intestine, resulting in diarrhea. This type of diarrhea can be caused by various drugs or by stress. Whatever the cause, diarrhea results in the loss of fluid and other substances from the body. If the diarrhea affects the small intestine, necessary nutrients will be lost with the fluid. As stated in Chapter 6, bicarbonate is released into the small intestine to neutralize hydrochloric acid from the stomach. If the bicarbonate is not reabsorbed, the acid-base balance of the blood and tissues will be affected. The person’s tissues will become more acidic, and body proteins will not function normally. Some of the substances lost as a result of diarrhea are electrolytes. Two principal electrolytes are sodium and potassium. Loss of these chemicals causes electrical imbalances in the body that affect heart and nerve function. If the loss is severe enough, heart function will be compromised and the central nervous system will cease to work efficiently.

3

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dIgestIon and nutrItIon

Perhaps the most damaging effect of diarrhea is the loss of water. As water is lost, it is drawn from the body tissues to keep the blood liquid, eventually causing the body tissues to lose function and shut down completely, resulting in death.

WhAT feCeS TeLL US AboUT The boDy Feces that have been eliminated from the body can be used to obtain information about the health of the person. As stated above, the color of feces and the presence of increased fat can indicate gallbladder problems. This material can also be used to screen for a type of malignancy called colorectal cancer. This form of cancer causes bleeding into the digestive tract that can be detected in the feces. If colorectal cancer is

YOUR HEALTH: FOOD POISONING Food poisoning occurs when bacteria in food are not killed before being ingested. For example, if food is left out in the summer heat, particularly anything containing dairy products, it provides an ideal growth medium for staphylococcus bacteria. If food is kept refrigerated, the bacteria cannot grow and produce the chemical toxins that cause symptoms. Improper handling of food may also cause bacterial contamination. If chicken or fish, which usually harbor salmonella, are not cooked properly before they are eaten, people may become ill. also, if the infected chicken or fish is prepared on the same surface as other foods without that surface being adequately cleaned, the bacteria can be transmitted to foods that might not be cooked to the same high temperatures as chicken. salmonella has a 12- to 24-hour incubation period before symptoms of severe diarrhea appear. the microorganism frequently damages the walls of the intestinal tract to the point that it can get through the lining cells and gain access to the rest of the body. When this happens, the person develops an infection of the bloodstream and possibly other organs of the body. this type of infection can be life-threatening.

the Large Intestine and elimination

detected early enough, it can be eliminated, saving the person’s life. Colorectal cancer is the second leading cause of death in men and the third leading cause of death in women.

ConneCtions Material to be digested enters the large intestine as chyme and leaves as feces. The digestive tube of the large intestine is divided into four major parts: the cecum, colon, rectum, and anal canal. Along the way, bacteria finish digesting any nutrients that were not broken apart and absorbed in the small intestine. These bacteria are normal and produce vitamins that the body needs, especially vitamin K for blood clotting.

5

8 Common Health Problems There are many conditions that affect the way the body digests food. Two common conditions are lactose intolerance and malabsorption.

Lactose Intolerance Because Amy has a form of lactose intolerance, she cannot digest milk unless it is in the form of yogurt, hard cheese, or cottage cheese. Drinking any milk products that contain lactose, such as that chocolate shake she had with her lunch, will result in discomfort. Lactose, the carbohydrate component of milk, must be broken into its two monosaccharides, glucose and galactose, to be absorbed in the small intestine, most often in the jejunum. As babies, most people produce lactase, the enzyme that digests lactose. This is important because the breast milk of humans contains more lactose than that of any other mammal. Many people, however, lose the ability to produce lactase as they become adults. These people can no longer tolerate milk or milk products except those that have been partially broken down, such as yogurt. In the United States, about 90% 76

Common Health Problems

of Asian Americans, 75% of African Americans, 50% of Hispanics, and 20% of Caucasians are lactose intolerant. Amy makes some lactase, so some of the sugar from the shake will be absorbed in the intestine. The severity of the symptoms that Amy experiences depends on how much lactase her body makes, because total absence of the enzyme is extremely rare. A short time after drinking the shake, she may experience diarrhea, gas, bloating, and abdominal cramps. The sugar is retained in the small and large intestines and causes more water to be retained in the digestive tube than normal. This accounts for the bloating feeling in the abdomen. The lactose that cannot be used by her body is used by the bacteria in her intestine. These bacteria ferment the sugar and produce hydrogen and carbon dioxide gases. The bacteria also produce acid, which affects abdominal sensory nerves and causes the abdominal cramping. Anyone with lactose intolerance should avoid milk, milk solids, whey (the liquid from milk), and casein, which is milk protein. Lactose is also found in breads, cereals, instant soups, instant potatoes, salad dressings, and nondairy powdered creamers. Drinking acidophilus milk or taking a pill containing lactase can also help avoid the digestive problems. In addition, about 20% of prescription drugs and 5% of overthe-counter drugs contain lactose. People with lactose intolerance need to be careful not to become deficient in calcium or riboflavin, a B vitamin. Biologists suggest that the persistence of the production of lactase into adulthood by some people may have occurred because of the development of dairy farming thousands of years ago. People whose ancestors depended on dairy farming tend to continue to be able to consume milk all their lives.

Malabsorption Lactose intolerance is one of type of malabsorption syndrome, a collection of conditions that cause problems in get-

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ting nutrients from the digestive tract to the cells of the body. There are four kinds of malabsorption problems: (1) There can be a problem with the absorption of only one type of nutrient, such as lactose; (2) a problem with the production or delivery of gastric juices into the stomach; (3) a problem with the production or delivery of enzymes from the pancreas or bile from the gallbladder to the small intestine; or (4) congenital or developmental problems with the part of the circulatory system that transports absorbed water-soluble nutrients to the liver for processing (see Chapter 6). If there is something wrong with this part of the circulatory system, nutrients will not be used properly. Some forms of cancer and some parasitic infections can also cause these transport problems. Malabsorption syndromes lead to deficiencies in nutrients, primarily in proteins and lipids. The combination of inadequate amino acid absorption and insufficient iron results in iron deficiency anemia. Long-term malabsorption will cause a deficiency in vitamin B12, which also causes anemia. As stated in Chapter 6, the liver makes most of the proteins found in blood. If the liver does not get enough building blocks for the proteins, their concentrations, especially of albumin, will decrease. Albumin is an important blood protein in maintaining osmotic pressure between the blood and tissues. If the albumin levels get too low, water will leave the blood and pool in body cavities. If lipids are not absorbed properly, the volume of stool increases, and it becomes frothy and very foul smelling, a condition called steatorrhea. If the pancreas does not produce enough lipase, triglycerides are not broken down, and they remain in the intestines and are excreted as part of the feces. If the gallbladder does not contribute bile to the small intestine, micelles are not formed from cholesterol and long-chain fatty acids, and these chemicals are not absorbed. If fats are not absorbed, neither are the fat-soluble vitamins A, D, E, or K. As a result, deficiencies of vitamin A can result in night blindness; inadequate vitamin D will lead to decreased calcium

Common Health Problems

absorption and eventually to weakened bones; vitamin E deficiency can cause damage to cells from chemicals produced in metabolism; and low levels of vitamin K can lead to bleeding due to a deficiency of blood-clotting factor. The symptoms of malabsorption syndromes are similar and include weight loss, anemia, diarrhea, and abdominal distress. Children who suffer from malabsorption may not grow to their full potential height due to inadequate nutrients during growth spurts. In very young children, malabsorption may lead to a general failure to grow and develop normally. Therapy for malabsorption conditions depends on the cause of the problem. If there is an underlying disease or abnormality, it must be addressed and the malabsorption will be eliminated. If the malabsorption cannot be cured, supplements of vitamins and trace minerals like calcium, magnesium, and iron are used. Substitutions can be made for the triglycerides that cannot be absorbed. Short- and mediumlength fatty acids can be absorbed without being made into micelles.

Connections Different conditions can affect the processing and availability of food. When specific types of nutrients, such as lactose, are not digested or absorbed, the body experiences problems, such as diarrhea and possibly nutritional deficiencies.

79

9 Guides to Healthy Eating Although the body needs nutrients from a wide variety of foods, it does not need them in all the same amounts. For example, the body requires more carbohydrates than fats. For many years, there have been different recommendations about what people should eat to maintain a healthy diet. As experts have learned more about nutrition and the human body, the recommendations have changed accordingly.

Obesity An estimated 60% of Americans are overweight and 25% of them are obese. Obesity can be measured in several ways. One method states that a person who weighs more than 20% over the ideal weight according to the height/weight chart is obese. This method does not take into account an athlete who may weigh more than a nonathlete because of muscle, instead of fat. Another method of measuring obesity is to measure percentage of body fat. If a person’s body fat is more than 25% for a man or 30% for a woman, that person is considered obese. A third method bases obesity on a calculation called the body mass index (BMI). This index calculates a 80

Guides to Healthy Eating

ratio of body weight to height and attempts to adjust for body size, as follows: BMI

=

Body Weight in Pounds x 705 (Height in Inches)2

Normal = < 25, Overweight = 25–30, Obese = > 30 According to the World Health Organization (WHO), obesity has become a worldwide problem that has significant effects on health. Problems that were once considered limited to developed or industrialized countries now affect everyone. Because of obesity, the incidence of diseases such as heart disease, type 2 diabetes mellitus, and hypertension has increased around the world. Obese individuals are also prone to pulmonary disease, varicose veins, and gallbladder disease. They have an increased risk of breast, uterine, and colon cancers. There are many reasons why people may become obese. A small number of people are genetically programmed to convert nutrients to fat, no matter what. These people probably eat less than normal and still gain weight. Another group of people cannot control their eating, sometimes consuming 20,000 calories at one meal. An even smaller group of people have a very low metabolic rate because of thyroid problems and cannot metabolize nutrients properly. The incidence of obesity has increased for a variety of reasons. As modern conveniences in the home and workplace have spread, so has a more sedentary lifestyle. People ride or drive rather than walk. Many people have desk jobs instead of doing manual labor. More people watch sports instead of participate in them. Many others have to schedule time to exercise, instead of it being a natural part of their lives. Over many years, fat has become a principal component of people’s diets. In the past, humans developed methods of conserving fat to survive possible famines. Although people

81

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DIGESTION AND NUTRITION

have increased their consumption of it, they still have metabolisms that conserve fat whenever possible. Diets high in fat are valued in developing countries and have replaced local diets that have been in place for centuries. The simplest way to prevent being overweight or obese is to eat less and exercise more. Some weight reduction programs have helped people lose weight through behavioral modification, which involves psychological counseling and exercise to achieve weight loss. Eating a very low-calorie diet can also help people lose weight. These programs usually last about twelve weeks and must be conducted under medical supervision. Some people resort to diet suppressant drugs, but nearly all of these drugs have been found to have harmful side effects and should not be taken for weight loss. Diuretics provide short-term weight loss through loss of water; however, this can lead to dehydration, potassium loss, and central nervous system complications. The weight is immediately regained when the person is rehydrated. A more radical method of weight loss involves surgery. This method should only be used when the person is morbidly obese, meaning more than 100 pounds (45 kg) overweight, and when other methods have been unsuccessful. Several surgical interventions exist. All of the procedures either limit the amount of food that can be taken into the digestive tract or limit the absorption of nutrients once the food gets into the system. In one procedure, an adjustable gastric band can be placed around the stomach. This band can be tightened or loosened as needed to restrict the size of the stomach. A more radical procedure, called a gastric bypass, involves stapling part of the stomach to make a smaller pouch and attaching a segment of the small intestine to this pouch (Figure 9.1). This method limits both the amount taken into the stomach and the amount of nutrients that can be absorbed through the small intestine. A third surgical method, called vertical banded gastroplasty, makes a small stomach within

Guides to Healthy Eating

the stomach, which restricts the amount that can be eaten. If too much is ingested, the person vomits it back out. Actual removal of fat is achieved through liposuction and plastic surgery. These last two methods do not affect future eating, but remove existing fat.

FOOD PYRAMIDS AND DIABETES By the late 1980s, the increased amount of fat in Americans’ diets was causing an increase in heart attacks and strokes. In 1992, the federal government released nutritional recommendations in the form of the Food Guide Pyramid (Figure 9.2a). In 2006, the U.S. Department of Agriculture (USDA) published revised recommendations entitled “MyPyramid” (Figure 9.2b). This program provides consumers with help in establishing their own caloric needs and choices for healthy eating. The result is similar to the earlier food pyramid that was proposed in 1992, but is oriented as a triangular pie chart that is color coded for classifications of food, such as grains, vegetables, fruits, oils, milk, and meats and beans. The program also emphasizes personal activity, moderation, eating proportionately, variety in the diet, and gradual improvement. Sample diets tailored to an individual can be developed through an interactive component. Diabetes is a condition in which glucose does not pass normally from the blood into certain body tissues, and thus the glucose cannot be used to produce energy. There are two major types of diabetes: type 1 and type 2. Type 1 diabetes, which accounts for 10% of people with the condition, is a disease in which the body does not produce any insulin. Individuals with this condition must take insulin injections. Type 2 diabetes, which accounts for 90% of people with the condition, is a disease in which either the amount of insulin produced is inadequate or the insulin receptors on the cells do not work properly. In both cases, the result is that too much glucose remains in the bloodstream. Most people with type 2

83

84

DIGESTION AND NUTRITION

Figure 9.1 Gastric bypass surgery is a radical way to fight obesity by making the stomach smaller. Although drastic, it has worked for some people. It is only recommended for individuals who are more than 100 pounds overweight.

diabetes are overweight. The increased body weight and sedentary lifestyles of many individuals are believed to be significant contributors to the development of type 2 diabetes.

Guides to Healthy Eating

Figure 9.2a The original food pyramid created by the United States government urges Americans to eat from each of the five basic food groups each day. These groups include the bread, cereal, rice, and pasta group; the vegetable group; the fruit group; the dairy group; and the meat, poultry, fish, dry beans, eggs, and nuts group. The recommended numbers of servings are listed with each group, and fats, oils, and sweets are to be used sparingly.

Since the Food Guide Pyramid was first recommended, research at the Harvard School of Public Health has revealed that the government recommendations were based on some faulty assumptions, and thus the Harvard researchers developed still another food pyramid (Figure 9.2c). The original pyramid recommendations were based on the assumption that all fats and carbohydrates are equal: All fats are bad, and all carbohydrates are good. In reality, some fats may actually improve health, while many carbohydrates create conditions that ultimately harm the body. The Harvard alternative pyramid is called the Healthy Eating Pyramid. This guide takes into account the differences

85

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DIGESTION AND NUTRITION

Figure 9.2b “MyPyramid” provides consumers with some general guidelines for healthy eating.

among types of fats and carbohydrates. The new pyramid focuses on individual foods and is designed for lifelong health, not short-term weight loss. Instead of food being measured in terms of servings, the pyramid focuses on the number of times a day the food should be eaten. Instead of four tiers, there are seven (daily exercise has been added).

Guides to Healthy Eating

Figure 9.2c A food pyramid designed by researchers at Harvard University suggests that Americans need to eat more whole grains, fruits, and vegetables and less red meat and refined grains.

Fats or oils derived from plants tend to have unsaturated fatty acids that are healthier than animal-derived fats that are high in saturated fatty acids. Saturated and trans fats contribute to the development of arteriosclerosis, or hardening of the arteries, which ultimately results in heart attacks and strokes. Fish oils, however, tend to be healthier than beef fats. Unsatu-

87



dIgestIon and nutrItIon

rated fats do not contribute to clogging our arteries. With the exception of oils from the palm plant or from coconuts, which are higher in saturated fats than other plant oils, unsaturated fats are healthier for the body. The original pyramid grouped all animal foods with nuts and dry beans and made no mention of plant oils. The recommendations were an effort to decrease fat intake, which was good, but they also decreased the intake of helpful fats, which was harmful. Carbohydrates create an interesting problem. When complex carbohydrates are broken down and absorbed from the digestive tract, their presence in the blood stimulates the release of insulin from the pancreas. Insulin facilitates the transfer of sugar (glucose) from the carbohydrates into the liver, muscle, and body fat. Once inside these three types of tissues, the sugar is metabolized or stored. If the body has more sugar than is needed for energy, the sugar is stored, mostly as triglycerides (see Chapter 2), a principal component of body fat. The more sugar a person eats, the fatter the

DID YOU KNOW? Food prepared with unsaturated fats spoils faster than food prepared with saturated fats, so food manufacturers prefer to use saturated fats in their products. Many consumers are aware of the differences between the two types of fatty acids and prefer to buy products that they feel are healthier. thus, some food manufacturers no longer prepare their products with saturated fats. other companies have produced a modified unsaturated fat called a partially hydrogenated fat in which extra hydrogen atoms have been added to unsaturated fatty acids, converting them to a saturated form, but retaining the original chemical name indicating an unsaturated product. these altered fats can be found in some brands of shortening, margarine, and other products that were once derived from animal products.

guides to Healthy eating

person gets. The more glucose the body has in the blood, the higher the levels of insulin released to handle the sugar. If a person eats a meal that gets glucose into the blood rapidly, the person’s blood glucose concentration increases quickly with a corresponding high insulin level. The current theory on the development of type 2 diabetes states that if these spikes of glucose and insulin occur frequently, the liver, muscle, and fat tissues may lose sensitivity to insulin. Thus, the body can no longer eliminate glucose from the blood adequately, resulting in the development of diabetes. Different types of carbohydrates are broken down and absorbed at different rates from the digestive tract. Whole grains break down slowly, while refined grains break down quickly, flooding the blood with glucose. As stated above, the rapid increase in the concentration of glucose in the blood may contribute to the development of diabetes. In addition, it also causes more of the glucose to be converted to fats for storage. This fat storage contributes to obesity if the energy is not used in exercise. The original pyramid did not differentiate between these types of carbohydrates. The average American diet is now fairly close to the federal government recommendations, but with a heavier emphasis on animal products than is recommended. Daily, a person eats about seven servings of bread, cereal, rice, or pasta; about five servings of meat, fish, poultry, eggs, nuts, or dry beans; three servings of vegetables; and three servings of milk, yogurt, cheese, and fruit. Fats and sweets have risen to the top tier.

gLyCeMiC iNDex To determine which carbohydrates release glucose quickly and which ones release it slowly, more than 300 foods have been evaluated and put on a scale called the glycemic index . The higher the food is on the scale, the more quickly its glucose enters the bloodstream and the higher the spike of insu-



90

DIGESTION AND NUTRITION

lin that results. Such insulin spikes increase the probability of developing diabetes and obesity. The lower the food is on index, the more slowly it raises the blood sugar level and the more gradual the increase of insulin. The glycemic index was

Table 9.1 Glycemic Index Examples food

glycemic index

Rice (instant)

91

Corn flakes

83

Cheerios

74

Bagel

72

Saltines

72

Macaroni and cheese

64

Raisins

64

Rye bread

64

Banana

62

Danish pastry

59

Oatmeal cookies

55

Orange juice

55

Pita bread

54

Pound cake

54

Oatmeal cereal

53

Ice cream

50

Rice (parboiled)

47

Macaroni

46

Baked beans

43

Grapes

43

Spaghetti

40

Apple

38

Yogurt

38

Milk

34

Chickpeas

33

Guides to Healthy Eating

originally created to help patients with diabetes control their blood sugars. The index is useful to anyone wishing to choose among the options of the Healthy Eating Pyramid (Fig 9.2c). Glucose is rated at 100.

Fast-Food Dilemmas Amy’s lunch at the fast-food restaurant of burger, fries, and chocolate shake filled her up, but with what? Amy’s hamburger was made of a 100% beef patty along with the bun, ketchup, mustard, pickles, onions, salt, and pepper. The fries were made of potatoes, partially hydrogenated soybean oil, natural beef flavor, dextrose, and sodium acid pyrophosphate. The chocolate shake contained whole milk, sucrose, cream, nonfat milk solids, corn syrup solids, mono- and diglycerides, guar gum, imitation vanilla flavor, carrageenan, cellulose gum, and vitamin A palmitate. The chocolate syrup in the shake was made from high fructose corn syrup, “regular” corn syrup, water, processed cocoa, natural and artificial flavors, salt, potassium sorbate, and vanillin. In all, Amy consumed 1,190 calories for lunch with 33.6% of the fat as saturated fats. She had 75 milligrams of cholesterol, 182 grams of carbohydrates, 29 g of protein, and 990 mg of sodium in the meal (Table 9.2). If she had substituted a small-size diet soda for the shake, she would have consumed a total of 610 calories, 25 mg of cholesterol, 80 g of carbohydrates, and 760 mg of sodium. Change the medium fries to a small size and the total calories drop to 480, the carbohydrates to 64 g, and the sodium to 680 mg. The number of calories and the amount of fats and carbohydrates in this meal may be acceptable occasionally, but eaten regularly, they can cause long-term harm. In March 2003, the WHO and the United Nations Food and Agriculture Organization (FAO) released a report stating that the ingestion of large amounts of sugars has become a worldwide problem. It is no longer confined to the developed countries, but has spread to the developing countries,

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DIGESTION AND NUTRITION

largely due to increased urbanization and modernization of traditional diets. Diseases such as heart disease, stroke, and diabetes that were once considered limited to industrial countries have become worldwide epidemics. The report recommended that people change their daily eating habits and increase their levels of exercise. The organizations suggested that people reduce the intake of energy-rich foods that are high in saturated fat and sugar, lower the amount of sodium (salt) in their diet, increase the fruits and vegetables, and exercise regularly. Their recommendations are close to the Healthy Eating Pyramid discussed earlier.

Fad Diets Currently, there are many fad diets being advertised. Some recommend eating all protein, while others recommend special drinks or pills. No matter what the fad diet, unsound weight loss programs tend to have some similar characteristics. They promise dramatic weight loss over a short period of time; they recommend eating an extremely low number of calories, usually without medical supervision; and they fre-

Table 9.2 Nutritional Facts Hamb.

Med. Fries

of

Fast Foods

Small Fries

Choc. shake

Diet Soda

Calories

250

360

230

580

0

Total Fat

9g

18 g

11 g

14 g

0g

Trans Fat

0.5 g

0g

0g

1g

0g

Sat. Fat

3.5 g

2.5 g

1.5 g

8g

0g

Sodium Protein Cholesterol Carbohydrates

520 mg 220 mg 140 mg 250 mg

20 mg

12 g

4g

3g

13 g

0g

25 mg

0 mg

0 mg

50 mg

0 mg

35 g

45 g

29 g

102 g

0g

Guides to Healthy Eating

quently try to make adherents to the diet depend on certain foods, usually provided by the program for a fee. In addition, many of these diets do not include any exercise regimens. People who use these diets to lose weight almost always gain the weight back because the changes the diets suggest are short-term changes that do not affect the person’s life on a long-term basis. Several diets suggest a diet high in proteins and fats and low in carbohydrates. These diets will result in immediate weight loss, but mostly from water loss. Any decrease in caloric intake will result in the body losing water during the first several days. This can account for the loss of several pounds, but the weight will return immediately when normal hydration is restored. These diets also cause the body to increase its production of ketones that will cause the body’s pH to become more acidic and cause significant problems in diabetic individuals. Ketones are produced whenever fats are burned. If fats are used slowly, the body can handle the resulting low ketone levels. Some of these diets suggest food combinations that they claim will either accelerate weight loss, or cancel each other out in the intestine. Both of these concepts are false. Each type of food is handled separately in the mouth, stomach, and intestine. Before going on any diet, individuals should consult their physicians about their ideal body weight and the best way to achieve that weight.

Anorexia Nervosa Anorexia nervosa is a disease approaching epidemic propor-

tions in the United States. It is estimated that as many as 7 million women and 1 million men have the condition. It affects minorities and people of all socioeconomic levels. According to the National Association of Anorexia and Associated Disorders (ANAD), 86% of people with anorexia

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4

dIgestIon and nutrItIon

EATING HEALTHY IN RESTAURANTS although it may be easy to read labels for food cooked at home, it is more difficult to learn about the nutritional content of food when eating in a restaurant. there are choices, however, for making meals healthier when eating out. For example, ordering and eating smaller portions is one way of eating healthy. substituting lower-calorie foods, such as fruit or vegetables, for fries or heavy side dishes, is another option. avoid eating bread before a meal. the bread may taste good, but it is high on the glycemic index, low in fiber, and low in other needed nutrients. ask for sauces on the side, and then use exactly the amount wanted. ask about how food is cooked and whether or not the sauce is made from cream. Cream sauces taste good, but they are not healthy. If possible, avoid fried foods. these also taste great, but some of the oil used in cooking is always retained in the food and adds a lot of calories and grams of fat to the meal. Consider ordering two appetizers instead of a large main course. this alternative will be cheaper, and result in lower calories and fats. If the main course is large, do not force yourself to eat it all. Bring half of it home for a later meal. this both halves the cost of the meal and halves the calorie and fat intake. Instead of ordering a big dessert, share one or a small number of desserts with the whole table. to determine portion sizes, use the following criteria: •

3 ounces of meat, fish, or poultry is about the size of a deck of cards.

•

one cup of vegetables is the size of a fist.

•

a cup of fresh fruit equals the size of a baseball.

•

Half a cup of rice is half a tennis ball.

•

one medium potato is equal to the size of a computer mouse.

•

a teaspoon of butter is equal to the tip of the thumb.

•

an ounce of cheese equals 4 dice.

guides to Healthy eating

report that the condition began before they reached age 20, and 10% report onset before 10 years of age. Two-thirds of sufferers say that it lasts up to 10 years. Outpatient treatment may exceed $100,000 per case. Some of the warning signs of the syndrome include: selfinduced starvation in the face of significant weight loss and fear of gaining any weight; compulsive exercise (the person may actually be a successful athlete); sensitivity to cold (loss of body fat decreases insulation in the body); absent or irregular menstruation; hair loss or excessive body hair. The person who has anorexia may be described as a perfectionist. Everything must be perfect for that person, especially the way his or her body looks. To that person, the body can never be too thin, and, in fact, he or she will view the body as overweight when it is obviously not. In the face of real starvation, the person is always hungry, frequently obsessed with food, but fights the impulse to eat. That person may prepare fancy meals for others, but appear to just pick at the food. Experts describe anorexic patients as having low self-esteem and as being depressed. To be diagnosed with anorexia, a person must be below 85% of his or her ideal weight, have an intense fear of weight gain (even when underweight), have a distorted view of his or her body weight or shape, and, if female, have missed three successive menstrual periods. People who suffer from anorexia nervosa will be malnourished, which will affect most of their body functions, including their ability to grow, heal cuts or bruises, and fight infections. They may have trouble sleeping, be chronically fatigued, and display moody behavior. Eventually, they will lose bone mass to the extent that they will have osteoporosis. If they survive and the condition continues, patients will exhibit early aging. About 6% of anorexic patients die, mostly from heart problems caused by low potassium levels.

5



dIgestIon and nutrItIon

INFORMED CHOICES ABOUT EATING the first and most important aspect in eating a healthy diet is learning about food. reading the nutritional information on food labels is an important way to learn how many calories the food contains and the distribution of fats, carbohydrates, and other substances. the federal government has set strict definitions for 12 terms that are used frequently on food labels, including free, reduced, lean, less, light, extra lean, low, fewer, high, more, good source, and healthy. the Food and drug administration (Fda) has also defined several health claims that can be used to describe food, as follows: •

“High protein” must have at least 10 g of protein per serving.

•

Food described as being “a good source of calcium” must have at least 100 mg of calcium per serving.

•

Food with “more iron” means that it has at least 10% more than the minimum daily requirement.

•

“Low fat food” means it contains 3 g or less per serving.

•

“reduced calorie” or “fewer calorie” foods must have at least 25% fewer calories per serving than a reference food.

•

“sugar free” foods cannot have more than 0.5 grams of sugar per serving.

•

“Light” may mean one-third fewer calories or half the fat of a reference food, or a 50% reduction in sodium.

guides to Healthy eating

ConneCtions Healthy eating requires eating certain foods in the correct proportions that the body needs. The incidence of type 2 diabetes, heart attack, and stroke in the United States can be attributed to the higher incidence of obesity and unhealthy eating among Americans. Dietary guides, such as the Food Pyramid and the Healthy Eating Pyramid, provide information on how to eat healthy.



Appendix: Conversion Chart

UNIT (METRIC)

METRIC

TO

ENGLISH

ENGLISH

TO

METRIC

LENGTH Kilometer

km

1 km

0.62 mile (mi)

1 mile (mi)

1.609 km

Meter

m

1m

3.28 feet (ft)

1 foot (ft)

0.305 m

Centimeter cm

1 cm

0.394 inches (in)

1 inch (in)

2.54 cm

Millimeter

1 mm 0.039 inches (in)

1 inch (in)

25.4 mm

mm

Micrometer µm

(one-millionth of a meter)

WEIGHT (MASS) Kilogram

kg

1 kg

2.2 pounds (lbs)

1 pound (lbs)

0.454 kg

Gram

g

1g

0.035 ounces (oz)

1 ounce (oz)

28.35 g

Milligram

mg

(1/1,000 gram)

Microgram

µg

(one-millionth of a gram)

VOLUME Liter

L

1L

1.06 quarts

Milliliter

mL or cc

1 mL

0.034 fluid ounce (fl oz)

Microliter

µL

(one-millionth of a liter)

TEMPERATURE °C = 5/9 (°F – 32)

98

°F = 9/5 °C + 32

1 gallon (gal)

3.785 L

1 quart (qt)

0.94 L

1 pint (pt)

0.47 L

1 fluid ounce (fl oz)

29.57 mL

Glossary

Abdominal cavity Anatomical cavity below the diaphragm. Absorption The process by which nutrients pass through the walls

of the digestive tract and into the blood.

Acetyl group A two-carbon molecule made from pyruvic acid or

the breakdown of fatty acids; excess levels of acetyl groups lead to ketone production.

Acidophilus milk Milk product containing lactase, the digestive

enzyme needed to break apart the sugar lactose found in milk.

Adenosine diphosphate (ADp) Precursor molecule to adenosine

triphosphate.

Adenosine triphosphate (ATp) Molecule that provides energy used

by cells to perform metabolic processes.

Adipose tissue Fat, mostly triglycerides; functions as energy storage

and insulation to retain body heat.

Adrenal cortex Outer portion of the adrenal gland that produces

assorted steroid hormones.

Adventitia Connective tissue covering of the digestive tube; also

known as serosa.

Aerobic cellular respiration The pathways of cellular respiration that

require oxygen.

Albumin Primary blood protein that functions to control osmotic

pressure between blood and tissues and as a carrier of ions, drugs, and assorted chemicals.

Aldosterone Adrenal cortical hormone primarily responsible for the

reabsorption of sodium in the kidneys.

Amylases Enzymes responsible for the breakdown of starch;

produced in the salivary glands and the pancreas.

Amylopectin Highly branched form of starch. Amylose Unbranched form of starch. Anabolism Synthesis of complex biochemicals in the body from

simpler ones.



100 DIGESTION AND NUTRITION Anaerobic cellular respiration The pathways of cellular respiration

that do not require oxygen.

Anal canal Short section of the large intestine that ends at the anus. Anemia Inability of the blood to deliver an adequate amount of

oxygen to body tissues.

Anorexia nervosa Syndrome resulting in severe weight loss; a state

of starvation with associated tissue degeneration and damage.

Antibodies Proteins produced by lymphocytes (white blood cells)

that attack microorganisms and other foreign cells.

Appendicitis Inflammation of the appendix. Appendix Structure attached to the cecum of the large intestine. Atherosclerosis Development of fatty plaque deposits in the blood

vessels, primarily arteries. The plaque collects in the lumen of the vessel, restricting blood flow through the vessel. If a portion of the plaque breaks off and moves through the vessel, it may clog the vessel, damaging the surrounding tissue.

Basic metabolic rate (BMR) Rate at which the body expends energy

over a specific period of time.

Bile Substance produced by the liver, stored in the gallbladder, and

secreted into the small intestine; it increases the water solubility of digestive fats to facilitate absorption.

Bile salts Salts derived from cholesterol, found in bile; they increase

the solubility of digestive fats.

Bilirubin Breakdown product of the heme in hemoglobin, secreted

in bile.

Bolus Rounded mass of food and saliva that is swallowed. Brunner’s glands Glands found in the submucosa of the duodenum. Buccinator Skeletal muscle used in chewing. Calculus Another name for tartar. Calorie The unit of energy needed to raise the temperature of one

gram of water by one degree Celsius.

Capillaries Smallest vessels of the circulatory and lymphatic sys-

tems; sites of exchange of materials between the vessels and surrounding tissues.

Glossary Carbohydrate General term for sugar or starch or similar

compound.

Carboxypeptidase One of several pancreatic enzymes that break

down proteins.

Catabolism Breakdown of complex biochemicals in the body. Cecum Pouch of tissue at the juncture of the small and large

intestines.

Cellular respiration The process by which nutrients, mainly glucose,

are broken down to obtain energy for metabolism.

Cellulose Polysaccharide that is not digestible; fiber in the diet. Cementum Chemical that holds teeth in place. Chief cell A cell that makes pepsinogen, a precursor of pepsin that

assists in protein breakdown; found in the lining of the stomach.

Cholecystokinin (CCK) Hormone made in the duodenum that stimu-

lates gallbladder contractions and pancreatic secretions.

Cholesterol Lipid used to make steroidal hormones and found in

cell membranes, where it increases flexibility; excess cholesterol is related to increased risk of heart disease.

Chylomicrons Combinations of proteins and lipids that travel from

the digestive tract to body tissues; they are ultimately removed from the blood by the liver.

Chyme Material that leaves the stomach and enters the intestines. Chymotrypsin One of the pancreatic enzymes that break down

proteins.

Collagen Protein substance that gives strength to tissues. Colon Longest part of the large intestine. Colorectal cancer Cancer of the colon or rectum. Colostomy Removal of part of the colon and attachment of the end

of the large intestine to a hole made in the abdominal wall.

Columnar epithelial cell Type of cell found lining the respiratory and

digestive tracts.

Connective tissue Type of primary tissue containing cells, fibers, and

an intracellular matrix.

101

102 DIGESTION AND NUTRITION Covalent bond Strong chemical bond based on the sharing of elec-

trons around atoms in the bond; energy is needed to make or break this type of bond.

Crown The upper part of a tooth; the part that is visible above the

gums.

Defecation Process of elimination of feces. Dehydration Loss of water from the body resulting in increasing

density of cell cytoplasm and compromised cellular functions.

Dental plaque Buildup of material on teeth, frequently from bacte-

rial metabolism.

Dermatitis Inflammation of the skin. Dextrose Synonym for glucose, a common sugar. Diabetes mellitus Disease involving the clearance and proper

metabolism of glucose; type 1 involves deficiencies in insulin production; type 2 involves a decrease in the sensitivity of the cells to insulin.

Diaphragm Skeletal muscle that divides the thorax from the abdo-

men; contraction of the diaphragm causes the lungs to fill with air.

Digestion The physical and chemical breakdown of food into nutri-

ents that can be absorbed by the cells of the body.

Disaccharide Sugar made up of two monosaccharide molecules

joined together, e.g., sucrose, lactose, and maltose.

Diuretics Chemicals that cause increased urination. DNA (Deoxyribonucleic acid) Nucleic acid that contains the heredity

information.

Duodenum First section of the small intestine. Elastase A pancreatic enzyme that helps break down proteins. Electrolytes Substances, including sodium, potassium, chloride, and

bicarbonate, that help regulate the body’s metabolic processes.

Electron transport chain Series of oxidation and reduction reactions

that result in the production of ATP, using the energy contained in electrons.

Enamel Hard material that coats the outside of teeth. Enzymes Proteins that increase the rate of chemical

reactions.

Glossary Epiglottis Part of the larynx; covers the opening to the trachea dur-

ing swallowing.

Epithelium Tissue that covers or lines body organs or structures;

may be in single or multiple layers.

Essential amino acids Eight amino acids that must be included in

the diet because the body cannot synthesize them.

Essential fatty acids Fatty acids that must be included in the diet

because humans cannot make them.

Estrogen Reproductive hormone; produces secondary sex charac-

teristics in females.

Fad diets Diets promising significant weight loss, usually concen-

trating on one nutrient.

Fat Any organic chemical with no surface charges; fats are insoluble

in water; also referred to as lipids.

Fatty acid Chain of carbon atoms with hydrogen atoms attached;

may be saturated or unsaturated; part of triglycerides.

Food pyramid Method of organizing the diet to emphasize greater

intake of certain materials that appear at the bottom, and less of those that appear on the top.

Fundus The part of the stomach below the connection to the

esophagus and above the body.

Gallbladder Organ that stores bile. Gastrin Hormone secreted by the stomach lining; stimulates the

production of other stomach digestive juices, such as hydrochloric acid and pepsinogen.

Glucagon Hormone produced by the pancreas that increases blood

glucose levels; opposes the actions of insulin.

Gluconeogenesis Making glucose from other biochemicals, usually

from amino acids.

Glycemic index Calculation giving an estimate of the amount that a

food will increase blood glucose levels.

Glycerol Three-carbon compound; part of triglycerides. Glycogen Energy-compound storage found in animals; resembles

starch from plants.

Glycolysis Reaction pathway beginning the breakdown of glucose;

does not require oxygen.

103

104 DIGESTION AND NUTRITION Goblet cells Mucus-producing cells found in the respiratory and

digestive systems.

Growth hormone Pituitary hormone that regulates body growth up

to puberty and contributes to carbohydrate metabolism throughout life.

Heart attack Blockage of a coronary artery resulting in damage to

heart tissue and the compromise of cardiac function.

Heartburn Regurgitation of stomach contents, including hydrochlo-

ric acid, into the esophagus, causing damage to the lining of the throat.

Hemoglobin Chemical that carries oxygen in red blood cells. Hemorrhoids Swelling of blood vessels around the anus. High-density lipoprotein (HDL) Combination of proteins and choles-

terol frequently called “good cholesterol”; scavenges cholesterol from tissues and returns it to the liver for elimination in bile.

Hormones Chemicals released from glands in the body that control

tissue and organ functions.

Hydrogen bond Chemical bond based on the unequal sharing of

electrons, resulting in weak positive and negative charges on the surface of a compound.

Hydrogen ion Hydrogen atom that is missing an electron and has a

positive charge.

Ileum Last portion of the small intestine. Inflammation Normal body reaction to cell damage, usually from

the attack of a microorganism.

Ingestion The process of taking food into the body. Insulin Hormone that decreases the level of blood glucose and

increases the storage of energy for the body.

Intrinsic factor Substance produced by the parietal cells of the stom-

ach lining; needed for the absorption of vitamin B12.

Ionic bond Chemical bond formed by the attraction of positive and

negative ions.

Jejunum Middle portion of the small intestine. Keratin Protein that fills the skin cells and helps make the skin

waterproof.

Glossary Ketones Acidic chemicals produced when excess acetyl groups,

such as acetoacedic acid, beta-hydroxybutyric acid, and acetone, are present.

Krebs cycle One of the pathways in aerobic respiration. The Krebs

cycle accepts acetyl groups and cycles them through a series of reactions, breaking the acetyl group to CO2 and water. Highenergy electrons are stripped of intermediate chemicals for ATP production in the electron transport chain.

Lactose intolerance Condition in which the enzyme lactase, needed

to break down lactose, is not produced in adequate amounts; results in intestinal cramping and diarrhea.

Lamina propria Layer of the mucosa; the inner portion of the wall

of the digestive tube.

Laryngopharynx Part of the larynx extending from the back of the

mouth to the larynx.

Lingual lipase Form of lipase made in the salivary glands of the

mouth; helps break down dietary triglycerides.

Lipids Another term for fats. Liver Major organ of the abdomen, the body’s “chemical factory”;

makes blood proteins and clotting factors, processes carbohydrates, and detoxifies drugs and poisons.

Loose connective tissue Form of connective tissue found around

blood vessels.

Low-density lipoprotein (LDL) “Bad cholesterol,” made of proteins,

cholesterol, and triglycerides; it transports triglycerides to tissues and organs. Increased levels of LDLs are associated with increased risk of heart attack and stroke.

Lymph nodules Concentrations of lymphocytes found in tissues;

they help screen for foreign material to protect the body from microorganisms.

Lymphatic capillaries Blind-ended, smallest vessels of the lymphatic

system, they drain excess fluid from tissues and receive fat-soluble material from digestive tube epithelia.

Lymphocyte Type of white blood cell that is involved in the immune

response; produces antibodies.

Malabsorption Inability to absorb nutrients adequately; leads to

nutritional deficiencies.

105

106 DIGESTION AND NUTRITION Masseter muscle Muscle found in the cheek and used in chewing. Metabolism Sum of the anabolic and catabolic biochemical path-

ways in the body.

Micelles Mixture of lipids and bile salts that are absorbed from the

small intestine into the epithelial cells.

Microvilli Projections of the membranes of digestive epithelia that

increase surface area in the small intestine.

Minerals Metallic elements needed by the body, e.g., calcium, mag-

nesium, sodium, potassium, and iron.

Monosaccharide Simple sugar molecule; glucose, fructose, and

galactose are monosaccharides.

Mucosa Innermost layer of the wall of the digestive tube. Muscularis Third layer of the wall of the digestive tube, made of

smooth muscle.

Muscularis mucosa Thin layer of smooth muscle, part of the

mucosa.

Nasopharynx Portion of the pharynx found at the back of the nasal

passage.

Nitrogen balance Determination of an adequate amount of amino

acids to support body growth and development, expressed as nitrogen content.

Nucleic acid Form of a biochemical that the body uses to store and

access genetic information.

Nucleotide Basic building block of nucleic acids; contains a nitrog-

enous base, phosphate, and a sugar.

Obese Grossly overweight; limits may be set by different criteria. Orbicularis oris Skeletal muscle that makes up the lips. Oropharynx Portion of the pharynx found at the back of the mouth. Osmotic pressure Balance of dissolved material on both sides of a

membrane that controls the passage of water between cells and their surrounding tissues.

Osteoporosis Abnormal process resulting in a decrease of bone

density.

Glossary Oxidative phosphorylation Process used by the electron transport

system to generate ATP from the energy in electrons derived from the Krebs cycle.

Pancreas Organ that produces hormones related to glucose metabo-

lism (insulin and glucagon) and digestive chemicals.

Pancreatic lipase Pancreatic enzyme that breaks down triglycerides. Papillae Structures on the top surface of the tongue, some of which

contain taste buds.

Parietal cells Cells found in the stomach epithelia that produce

hydrochloric acid and intrinsic factor.

Partially hydrogenated fat Fatty acids that have had hydrogens

added; similar to saturated fats.

Pepsin Digestive enzyme made from pepsinogen; begins the diges-

tion of proteins.

Pepsinogen Precursor to pepsin, made in the stomach by chief cells. Peptide Short chain of amino acids. Periodontal ligament Ligament that connects a tooth to the jaw. Peristalsis Waves of contractions of smooth muscles that move

material through the digestive tube.

Peritonitis Inflammation of the peritoneum, which is the membrane

that covers the organs in the abdominal cavity.

Pernicious anemia Deficiency of intrinsic factor, resulting in vitamin

B12 deficiency.

Peyer’s patches Lymphoid tissue; serve as an anatomical marking

for the ileum.

pH Scale from 1 to 14 measuring the degree of acidity or alkalinity.

A pH of 1 to 6.9 is acidic, 7.1 to 14 is alkaline, and 7 is neutral.

Pharynx The passage between the mouth and the esophagus; has

three parts: naso-, oro-, and laryngopharynx.

Phospholipids Mixture of phosphates and fatty acids that make up

most of cell membranes.

Phytosterol Type of plant lipid analogous to cholesterol; cannot be

absorbed by animals.

107

108 DIGESTION AND NUTRITION Plaque Buildup of material on teeth; part food residue, part

bacteria.

Polysaccharide Multiple monosaccharides linked together, such as

starch, glycogen, and cellulose.

Proteins Complex molecules made of linked amino acids. Pulp Part of the tooth located at the center of the crown; contains

nerves, blood vessels, and connective tissue.

Pyruvic acid End product of glycolysis; it is converted into lactic

acid or to an acetyl group, which is then broken down by the Krebs cycle.

Rectum End portion of the intestines, adjacent to the anus. Rugae Folds of the lining of the stomach that allow for expansion. Salivary glands Paired glands around the mouth that produce

mucoid or watery saliva.

Salmonella Infectious bacteria; frequent cause of food poisoning. Saturated fat Form of fatty acids that contain the maximum num-

ber of hydrogen atoms.

Secretin Hormone made in the duodenum; stimulates gastric secre-

tion and motility and pancreatic secretions.

Serosa Connective tissue covering of the digestive tube; also known

as adventitia.

Shigella Infectious bacteria; frequent cause of food poisoning. Small intestine Site where digestion is completed and nutrients are

absorbed into the blood and lymph.

Smooth muscle Type of muscle not under voluntary control; makes

up a significant part of the digestive tube wall.

Sphincter Circular smooth muscle; when constricted, it closes off

access to a portion of the digestive tube.

Squamous epithelial cell Flat cell; may be in a single layer or

stratified.

Staphylococcus Infectious bacteria; frequent cause of food poison-

ing, especially with dairy products.

Starch Polysaccharide made by plants for energy storage; composed

of many glucose units linked together.

Glossary Steatorrhea Increased fat in feces; may be a result of gallbladder

problems.

Steroids A type of lipid containing hydrocarbon rings. Stretch receptors Specialized neurons that monitor the stretch of the

digestive tube.

Stroke Rupture of a blood vessel causing bleeding in the cranium

and pressure on the brain.

Submucosa Second layer of the digestive tube wall, under the

mucosa; contains connective tissue, blood vessels, and nerves.

Substrate phosphorylation Synthesis of ATP using the energy left

over from a particular chemical reaction.

Tartar White, brown, or yellow-brown deposits on teeth; also

known as calculus.

Teniae coli Bands of smooth muscle in the large intestine. Testosterone Male hormone that stimulates sperm production and

is responsible for secondary male sexual characteristics.

Thyroid gland Gland found in the neck; controls rate of metabolism. Tonsils Lymphoid tissue found on the back of the tongue and at the

back of the mouth.

Trace metals Minerals that are required by the body in low

concentrations.

Trans fats Unsaturated fats that have been partially hydrogenated

and made into stiff, saturated-like fats.

Triglyceride Type of lipid consisting of glycerol and three fatty acids;

long-term energy storage compound in animals.

Trypsin Digestive enzyme; activates other pancreatic enzymes and

works on proteins.

Unsaturated fat Fatty acid without the maximum number of

hydrogens.

Villi One-millimeter structures found in the small intestine that

increase the surface area for absorption; enzymes needed for final digestive steps are found on the villi.

Vitamins Chemicals that must be ingested in low concentrations;

they facilitate enzyme functions.

109

Bibliography

Burtis, C.A., and E.R. Ashwood, eds. Tietz Textbook of Clinical Chemistry, 2nd ed. Philadelphia: W.B. Saunders Co., 2005. The Glycemic Index—Sample. Available online at http://www. btinternet.com/~johnharker/table3.htm. “Health for Life,” Newsweek (January 20, 2003): 44–72. Johnson, M.D. Human Biology. New York: Benjamin Cummings, 2007. Marieb, E.N. Human Anatomy & Physiology, 6th ed. New York: Benjamin Cummings, 2007. McDonald’s Nutrition Facts, Rev. ed. Revised January, 2008. Available online at http://www.mcdonalds.com. National Association of Anorexia and Associated Disorders. Eating Disorders. Available online at http://www.anad. org. Tortora, G.J., and S.R. Grabowski. Principles of Anatomy and Physiology. New York: John Wiley & Sons, Inc., 2008. Whitney, E.N., and S.R. Rolfes. Understanding Nutrition. New York: West Publishing Co., 1993. WHO Expert Report on Diet and Chronic Disease. Available online at http://www.who.int/mediacentre/releases/2003/ pr20/en/

110

Further Resources

American Dietetic Association. New Family Cookbook for People with Diabetes. New York: Simon & Schuster Adult Publishing Group, 2007. Castle, D., P.A. Singer, C. Cline, A.S. Daar, and C. Tsamis. Science, Society and the Supermarket: The Opportunities and Challenges of Nutrigenomics. Hoboken, N.J.: John Wiley & Sons, 2006. Chow, C.K., ed. Fatty Acids in Foods and Their Health Implications. Boca Raton, Fla.: CRC Press, 2007. Clark, N. Nancy Clark’s Sports Nutrition Guidebook. Champaign, Ill.: Human Kinetics Publishers, 2008. Dijkstra, A.J., R.J. Hamilton, and W. Hamm, eds. Trans Fatty Acids. Hoboken, N.J.: John Wiley & Sons, 2008. Di Pasquale, M.G. Amino Acids and Proteins for the Athlete: The Anabolic Edge, 2nd ed., Boca Raton, Fla.: CRC Press, 2007. Dosil, J. Eating Disorders in Athletes. Hoboken, N.J.: John Wiley & Sons, 2008. Driskell, J.A. ed. Sports Nutrition: Fats and Proteins. Boca Raton, Fla.: CRC Press, 2007 Duyff, R.L., and American Dietetic Association Staff. American Dietetic Association Complete Food and Nutrition Guide. Hoboken, N.J.: John Wiley & Sons, 2006. Giroux, I. Applications and Case Studies in Clinical Nutrition. Baltimore, Md.: Lippincott Williams & Wilkins, 2007. Irven, N., and P.L. Ritchie. Please Don’t Eat the Wallpaper!: The Teenager’s Guide to Avoiding Trans Fats, Enriched Wheat and High Fructose Corn Syrup. Garden City, N.Y.: Morgan James Publishing, LLC, 2008. Kabara, J.J. Fats Are Good for You: How Saturated Fat and Cholesterol Actually Benefit the Body. Berkeley, Calif.: North Atlantic Books, 2008. Keshav, S. Gastrointestinal System at a Glance. Hoboken, N.J.: Wiley-Blackwell, 2007. 111

112

DIGESTION AND NUTRITION

Maher, A.K., ed. Simplified Diet Manual. Iowa Dietetic Association ed. Hoboken, N.J.: John Wiley & Sons, 2007. McCracken, J., and L. Larsen. Everything Lactose Free Cookbook: Easy-to-Prepare, Low-Dairy Alternatives for Your Favorite Meals. Avon, Mass.: Adams Media Corp., 2008. McWilliams, M. Food Fundamentals. Upper Saddle River, N.J.: Prentice Hall, 2008. Ojeda, L. Safe Dieting for Teens. Alameda, Calif.: Hunter House, 2008. Platkin, C.S. Diet Detective’s Calorie Bargain Bible: More than 1,000 Calorie Bargains in Supermarkets, Kitchens, Offices, Restaurants, the Movies, for Special Occasions and More. New York: Simon & Schuster Adult Publishing Group, 2008. Rinzler, C.A., and M.W. Graf. Controlling Cholesterol for Dummies. Hoboken, N.J.: John Wiley & Sons, 2008. Scott, A., and E. Fong. Body Structures and Functions. Florence, Ky.: Thompson Delmar Learning, 2008. Sewell, R.W. and L. Rohrbough. Weight Loss Surgery with the Adjustable Gastric Band: Everything You Need to Know Before and After Surgery to Lose Weight Successfully. Cambridge, Mass.: Da Capo Press, 2008. Stuart, T. The Bloodless Revolution: A Cultural History of Vegetarianism from 1600 to Modern Times. New York: W.W. Norton & Co., 2008. Taylor, K. Going Hungry: 20 Writers on Desire, Self-Denial, and Recovering from Anorexia. New York: Knopf Publishing Group, 2008. Taylor-Butler, C. Food Pyramid. San Francisco: Children’s Press, 2008. Thompson, T. Gluten-free Nutrition Guide. New York: McGrawHill Co., 2008. Warshaw, H.S., and K.M. Bolderman. Practical Carbohydrate Counting. Alexandria, Va.: American Diabetes Association, 2008.

Further Resources

Weitzner, A. Ayudando a Personas Con Anorexia, Bulimia Y Comer Compulsivo: Guia Practica Para Maestros, Terapeutas Y Medicos. Santa Cruz, Atoyac, Mexico: Editorial Pax Mexico, 2008.

WEB SITES American Gastroenterological Association

www.gastro.org Digestive Disorders Foundation

www.digestivedisorders.org.uk Foodborne and Diarrheal Diseases—Centers for Disease Control and Prevention

www.cdc.gov/ncidod/dbmd/foodborne/index.htm Mayo Clinic—Diarrhea

www.mayoclinic.com/health/diarrhea/D500292 Movie of Stomach Functions

www.brainpop.com/search/?keyword=digestion NAMI: Anorexia Nervosa

www.nami.org/helpline/anorexia.htm National Digestive Diseases Information Clearinghouse

www.digestive.niddk.nih.gov/index.htm National Institute of Diabetes and Digestive and Kidney Diseases

www.niddk.nih.gov/index.htm A Voyage Through the Digestive Tract—Colorado State University

www.vivo.colostate.edu/hbooks/pathphys/digestion

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Index

A

absorption, 11, 32–34, 36, 62–64 acetyl groups, 29–30 adipose tissue, 19 ADP (adenosine diphosphate), 13, 26–27 adrenal cortex, 22 adventitia, 34, 37, 40, 49 aerobic cellular respiration, 27–29 albumin, 78 alcohol, 57 aldosterone, 22 α-linoleic acid, 20 amino acids, 17–19, 26, 34, 40 amylase, 47, 52, 54, 62 amylopectin, 16 amylose, 16 anabolism, defined, 26 anaerobic cellular respiration, 27 anal canal, 68, 72 anemia, 20 anorexia nervosa, 93–95 antibiotics, 58 anus, 70, 71 appendicitis, 68 appendix, 68 arteriosclerosis, 87–88 ascorbic acid. See vitamin C aspirin, 58 atherosclerosis, 72 ATP (adenosine triphosphate), 13, 26–27, 29, 30

B

B vitamins, 24–25, 54, 67, 77, 78 bacteria, 45–46, 58, 68, 70–74 basic metabolic rate (BMR), 27 beef, 16, 34 behavioral modification, 82 bicarbonate, 62, 73 bile, 20, 38, 61, 71 bile salts, 61, 64

bilirubin, 61, 71 bismuth salicylate, 58 blood capillaries, 59 blood clotting, 24, 79 BMI (body mass index), 80–81 bolus, 37, 38, 39, 41 bread, 94 Brunner’s glands, 59 buccinator muscle, 43

C

caffeine, 56–57 calcium, 25, 46, 79, 96 calories, 27 capillaries, 20, 22, 36, 59 carbohydrates, 15–17, 22, 31–33, 76–77, 88–91. See also starches; sugars carbon dioxide, 30 carboxypeptidase, 62 cardiac sphincter, 49, 50, 52 catabolism, defined, 26 cecum, 68 cellular respiration, overview of, 30 cellulose, 17 cementum, 45 chemical bonds, 12–13, 30–31 chewing, overview of, 41–49 chief cells, 54 cholecystokinin (CCK), 61–62 cholera, 73 cholesterol, 17, 19–22, 34, 40, 64–65 chylomicrons, 64–65 chyme, 38, 56 chymotrypsin, 62 collagen, 17 colon, 68, 70 colorectal cancer, 72, 74–75 colostomy, 71 columnar epithelial cells, 34, 36, 38, 54, 70 covalent bonds, 13, 30–31 crowns, 43–44

115

116

DIGESTION AND NUTRITION

D

defecation, 71–72, 74–75 defecation reflex, 72 dehydration, 82 dental hygiene, 46 dentin, 45 descending colon, 68 dextrose, 91 diabetes, 72, 83–85, 89 diaphragm, 49, 50 diarrhea, 34, 72–74 diet, proteins and, 18–19 diet drugs, 82 diets, fad, 92–93 digestion, defined, 11 digestive tract building blocks of, 40 hamburger and, 32 liver, gallbladder, pancreas and, 61 process of digestion and, 37–40 small intestine and, 57–59, 61–65 stomach and, 52–57 structure of, 34–40 dimethyl sulfide, 70 disaccharides, 15, 31, 32–34, 62–64 diuretics, 82 DNA, 62 duodenum, 57, 61

E

E. coli, 70, 73 elastase, 62 electrolytes, 65, 73 electron transport chain, 29, 30 empty calories, 17 enamel, 43–44, 46 energy, 12–13, 16, 26, 30–31 enzymes, 17, 39. See also specific enzymes epiglottis, 49 epithelium, 34–36 esophagus, 37, 40, 49, 50 essential amino acids, 18–19 essential fatty acids, 20 estrogens, 22 exercise, 82, 92, 95

F

fad diets, 92–93 fast food, 10, 91–92 fats. See also lipids beef and, 34 fad diets and, 93 healthy eating and, 87–88, 96 malabsorption syndromes and, 78 obesity and, 81–82 overview of, 19–20 vitamins and, 23 fatty acids absorption of, 64 amylase and, 47 as building blocks, 40 digestive tract and, 34 large intestine and, 71 oxidative phosphorylation and, 27 triglycerides and, 19 feces, 40, 74–75. See also defecation fiber, 17, 72 fibrous proteins, 17–18 folic acid, 25 Food Guide Pyramid, 83, 85 food poisoning, 74 food pyramids, 83–89 fructose, 15 fundus, 54

G

G cells, 55–56 galactose, 15 gallbladder, 39, 61, 71, 74, 78 gastric bands, 82 gastric bypass surgery, 82 gastric pits, 54 gastrin, 55–56 globular proteins, 18 glucagon, 61 gluconeogenesis, 16 glucose, 15, 29, 47, 56–57, 83–84, 88–89 glutamate, 45 glycemic index, 89–91, 94 glycerol, 19, 34, 40, 47, 64 glycogen, 16 glycolysis, 29

Index goblet cells, 36, 40, 47–48, 54, 70 goiter, 25 growth hormone, 18 gums, 46

H

hair, keratin and, 18 hard palate, 43 haustra, 70 HDL (high-density lipoprotein), 20–21 Healthy Eating Pyramid, 85–88, 91, 92 heart attacks, 20, 24, 87–88 heartburn, 50 Helicobacter pylori, 58 hemoglobin, 25 hemorrhoids, 72 hormones, 17, 38, 55–56 hydrochloric acid, 54–55, 56, 58, 62, 73 hydrogen bonds, 12–13

I

ileocecal valve, 68 ileum, 57 ingestion, overview of, 37–39 insoluble fiber, 72 insulin, 61, 88–89 intestinal fluid, 61 intrinsic factor, 54 ionic bonds, 12 iron, 25, 96

J

jejunum, 57, 59, 76 jugular vein, 59

K

keratin, 17–18, 43 ketones, 93 Krebs cycle, 29–30

L

lactase, 76–77 lactose intolerance, 10, 15, 34, 76–77 lamina propria, 34, 36

large intestine, 38, 40, 65, 67–75 laryngopharynx, 47 larynx, 49 LDL (low-density lipoprotein), 20–21 light foods, 96 lingual lipase, 47, 52 linoleic acid, 20 lipase, 62, 78 lipids, 19–20, 22, 40, 64, 78. See also fats liver, 16, 38, 59, 61, 67, 78 loose connective tissue, 36 low-calorie foods, 96 lymph nodules, 36 lymphatic capillaries, 36, 59 lymphocytes, 45–46

M

malabsorption syndromes, 77–79 maltose, 15 masseter muscle, 46 metabolism, overview of, 25–31 methane, 70 micelles, 64–65, 78 milk, 76–77. See also lactose intolerance minerals, 13, 22, 25 monosaccharides, 15, 31, 32–34, 40 mouth, 41–43 MSG (monosodium glutamate), 45 mucosa, 34–37, 40, 57–59, 70 mucus, 36, 38, 54, 58, 61 muscularis, 38, 40, 47–48, 49, 70 muscularis mucosa, 34-37, 40

N

nails, 18 nasopharynx, 47 night blindness, 78 nitrogen balance, overview of, 18–19 normal flora, 70–71 NSAIDs (nonsteroidal anti-inflammatory drugs), 58 nucleases, 62 nucleotides, 62–64 nutrients carbohydrates as, 15–17

117

118

DIGESTION AND NUTRITION digestive process and, 38–40 digestive tract and, 32 fats, lipids as, 19–20 overview of, 13–14 proteins as, 17–19 reasons for need for, 12–13 trans fats and, 21–22 nutritional status, determination of, 18–19

O

obesity, 80–83 oils, healthy eating and, 87–88 oleic acid, 20 omega-3 fatty acid. See α-linoleic acid omega-6 fatty acid. See linoleic acid orbicularis oris, 41–42 oropharynx, 41, 45, 47 osmotic diarrhea, 73 osmotic pressure, 78 osteoporosis, 95 oxidative phosphorylation, 27 oxygen, 27–28, 29

P

palate, 43 palmitic acid, 20 pancreas, 38, 61, 62 pancreatic lipase, 62 papillae, 45 parietal cells, 54, 56 parotid salivary gland, 46–47 partially hydrogenated fat, 88–89 pepsinogen, 54–55, 56 peptides, 62–64 periodontal disease, 46 peristalsis, 37-38, 50, 73 peritonitis, 58 pernicious anemia, 54 Peyer’s patches, 59 pH, 47, 50, 61, 93 pharynx, 37, 47–48 phosphates, 13 phospholipids, 19 phosphorylation, 26–27, 30 phytosterol, 22 plaque, 20

plaque (dental), 46 polysaccharides, 15–17, 31 portion size, 94 potassium, 73 proteins building blocks of, 34, 40 chemical bonds and, 31 diet and, 18–19 digestive process and, 34, 62 fad diets and, 92–93 healthy eating and, 96 lipid solubility and, 20–21 as major nutrients, 22 overview of, 17–18 pepsin and, 54–55 protons, 27 pulp, 45 pyloric sphincter, 57 pyruvic acid, 29–30

R

rapeseed oil, 20 rectum, 68 reduced calorie foods, 96 respiration, 27–29, 30 restaurants, 94 retinol. See vitamin A RNA, 62 roots, 45 rugae, 54

S

safflower oil, 20 saliva, 46–47 salivary glands, 46–47 Salmonella bacteria, 73, 74 saturated fats, 19–20, 87–88 secretin, 62 serosa, 34, 37, 40, 49 Shigella, 73 sigmoid colon, 68 small intestine, 38, 40, 57–59, 61–65, 70 smooth muscle, 36-37 sodium, 25, 73 soft palate, 43, 49 solubility, 19, 20–21, 23–25, 72

Index speech, 45 sphincters, 49 squamous epithelial cells, 36, 43, 47 Staphylococcus bacteria, 73, 74 starches, 15–16, 47, 52 stearic acid, 20 steatorrhea, 78 stercobilin, 71 steroids, 19, 22 stomach, 38, 52–57, 82–83 Streptococcus bacteria, 45–46 stretch receptors, 56 strokes, 20, 24, 87–88 subclavian vein, 59 sublingual salivary gland, 46–47 submandibular salivary gland, 46–47 submucosa, 34-36, 40, 59 substrate phosphorylation, 26–27, 30 substrates, defined, 26 sucrose, 15 sugars, 16, 17, 40, 91–92, 96 surgery, obesity and, 82–83 swallowing, 43, 49–51

T

tartar, 46 taste buds, 45 teeth, 41, 43–45, 46, 50 teniae coli, 70 testosterone, 22 thyroid, 25, 81 tongue, 45–46, 49 tonsils, 45–46 trace metals, 25 trans fats, 21–22 transverse colon, 68 triglycerides absorption of, 64–65 amylase and, 47 building blocks of, 40

carbohydrates and, 88–89 chemical bonds and, 31 digestive tract and, 34, 38, 40 LDL and, 20 lingual lipase and, 52 malabsorption syndromes and, 78 overview of, 19 oxidative phosphorylation and, 27 trans fats and, 21–22 trypsin, 62

U

ulcers, 56, 58 unsaturated fats, 19–20, 87–88 urinary tract infections, 70 uvula, 43

V

vegetable oils, 20 vertical banded gastroplasty, 82–83 villi, 59, 62–63 vitamin A, 78 vitamin C, 24–25 vitamin D, 23–24, 78–79 vitamin E, 79 vitamin K, 24, 67, 79 vitamins, 13, 22, 23–25, 67, 78–79. See also specific vitamins

W

water absorption of, 65 diarrhea and, 73–74 digestive process and, 39–40 diuretics and, 82 fad diets and, 93 hydrogen bonds and, 12–13 large intestine and, 67 stomach and, 57

119

About the Author

Robert Sullivan, Ph.D., MT (ASCP), is an associate professor

of medical laboratory sciences at Marist College in Poughkeepsie, New York. Dr. Sullivan teaches in both the medical laboratory science and the biology curriculums. His research interests include the toxic effects of heavy metals in alternative medicines, the use of medical laboratory assays to evaluate the nutritional status of athletes, and international issues in laboratory medicine.

120